One of the fundamental skills for a cook to develop is the control and use of heat. Heat is such a basic part of cooking that those of us who control it well take it for granted that everyone knows or should know how to use heat in the kitchen; those of us who don't control it at all probably don't even recognize the application of heat to food as a significant skill in and of itself. Chefs bristle and sometimes even grow angry when asked, "How long should I cook it?" It's an exasperating question to ask a cook, because there is no correct short answer to the question. Often you'll hear the flippant answer, "Until it's done." The chef who gives that answer knows that it isn't helpful to the person who asked, and yet he intends it in a helpful way: he wants to prod the questioner to think about the problem in a different way.
In an analogy that I thought was brilliant, an essay that I read a few months ago said that to tell someone to cook something for five minutes made as much sense as telling them that to get to your house they should drive for five minutes and then turn right. Anyone who drives knows that while the approximate time it will take to get somewhere can be known in advance, you don't use a timer to tell you when to take the next action (like turning right) as you drive. Instead, you look for landmarks or roadsigns. In the same way, a chef assumes that anyone who cooks will know that while the approximate cooking time of something can be known in advance, you can't use time to tell you when to take the next action (like adding the next ingredient, or deglazing your pan, or serving the finished dish) as you cook something. Instead you need to learn what signs to look for that the food being cooked has reached the state where it is time for the next step.
Most cookbooks encourage the idea that elapsed time is how to tell when something is cooked. Every step of a recipe that tells you to cook something also tells you how long to cook it. I've read that there are often battles between chefs and cookbook editors about whether to include timings in recipes or not. The editors and publishers always win the argument, and they are probably right to think that they will have a difficult time selling a cookbook that doesn't say anything about how long you should cook your onions before you add your tomatoes. Yet the fact remains that for many recipes, if you "cook over medium heat for five minutes" as instructed, whether the food you make comes out well or not is mostly a matter of luck (or your skill at knowing from the context and dish whether the author meant for you to saute or sweat the food), and not related to the quality of the dish or the recipe.
The more I cook, and the more I write about cooking and try to describe recipes to friends either in writing or while I'm talking to them, the more I believe that most of what you do in cooking, especially when you apply heat to something, is "goal-oriented." What I mean is that in every step of a recipe, the author has in mind some state that the food will be in at the end of that step. Usually the goal (the state in which the food should be) is not explicitly stated. Instead of telling you what the goal is, recipes generally give you an action to carry out, such as our earlier example, "cook over medium heat for five minutes." It's not only the "five minutes" part of that instruction that might or might not work out as intended; the "medium heat" bit leaves even more room for error. The cook would be better off if he were told, "Cook over medium heat until softened but still with some bite, and pale white or yellow in color, which might take about five minutes." Depending on your skill, how many other things you have going on in the kitchen at the same time, what kind of pan you use, how much heat you apply and how much you stir or shake the pan, that step might take anywhere from 3 minutes to 20 minutes. But if you focus on the goal -- soft texture and pale color -- you'll get the step right no matter how much time is involved.
It's difficult to write a goal-oriented cookbook that can be used by a general audience. One common approach is to include an introductory chapter or two in the book, before the chapters of recipes, with general information and instructions. But reality is that most people aren't ever going to read that stuff (though for me it is the most-read parts of the cookbooks I own). Another approach I've seen is to describe the goals of each step right in the recipe. One of the cookbooks that is among the handful of important books in my own development as a cook is Rick Bayless's Mexican Kitchen, and in it Bayless often takes a fairly long paragraph to describe each step. Since so many of the techniques are common to a lot of the recipes, he repeats the same description of a step several times throughout the book, so that when you go to make anything, you have everything you need to know right there in the recipe in front of you. As a result, many of his recipes take 3 pages or more of text, which can deter people from reading or attempting them. But it was a transformative book for me, because it started me along the road to thinking of every step in cooking a dish as having a desired outcome, and not just as a direction to be followed in the hope that everything would turn out well at the end of all of the steps.
Whenever you apply heat to food, you should know what your goal is. For example, when you cook anything in oil in a pan, you should at least know whether you want it to get browned or not, and in addition you might want to know whether you're trying to drive out none, some or most of its moisture. Recipes often don't specify these things, although they do sometimes throw you a hint about the color ("golden brown") you want to achieve, which can be helpful. In writing and describing recipes, I'm starting to think that describing at the beginning of the recipe the important intermediate goals to keep in mind as you cook is the way I like to tackle the issue. The "Cooking Objectives" section of the Potato Leek Soup recipe I posted here awhile ago is an example.
Once you know what your goals are, you can depart from the directions of your recipes and use your own methods to achieve them. Recently I read an article by Eric Ripert, the chef at Le Bernardin, regarded as the best restaurant in New York by many people (and where I ate the best piece of cooked salmon I've ever had), in which he outlined the most important aspects of the way he cooks a fish fillet. A couple of them were to heat the oil until it's smoking before you put in the fillet, and then put the fillet in skin-side down and press down on it firmly with a spatula to prevent the skin from curling. Not long after I read that, I saw an interview with Tom Colicchio, the founding executive chef (he has since moved on) of Gramercy Tavern, the restaurant where I had the best cooked striped bass (my "go-to" fish) dish I ever ate. He said in a nutshell, "You don't have to get your oil that hot, and pressing it down with a spatula like many chefs do is unnecessary." Who is right? Well, both are -- they can both produce perfectly-cooked fish, with crispy skin, using different techniques. Their goals are the same, their methods are different. But the lesson here is not that the technique doesn't matter, because you can't mix-and-match your favorite parts of each way of doing it: if you use very hot oil, you need to press with the spatula, or the fish will curl away from the pan and it won't cook correctly at all. I've used both methods successfully, and I don't really have a favorite, although based on how good the results have been a few times, if I needed to make a perfect fish fillet for a special plate, I'd go with the smoking oil and spatula. But again, to use that method, you need to know your goals and keep them in mind. You don't want to burn the fish, and when you use high heat often it's the fat (oil) that burns before the food does, and since the oil will move around in the pan and coat the food, burned oil will make the fish taste acrid and bitter even if you don't overheat its surface. The solution is to use a pan of the right size, that will just accommodate the fish you want to cook (with a half inch or a bit more between the pieces), but not any larger, because what burns your oil is having a large surface area of your pan over the burner without any food in contact with it to absorb the heat.