Believe it or not, one of the most common mistakes home cooks make is trying to cook their food before the pan is hot enough.
Fortunately, this is an easy problem to avoid, because all you have to do is absolutely nothing for the additional minutes your pan needs to reach the proper temperature.
Next time you're at a restaurant where you can see the line cooks from the bar or dining room, check out their pans. You'll probably see unused aluminum sauté pans on the back burners of the range with a little tongue of flame underneath (or maybe on a nearby flat top or grill). The flame's not up on high, but it's high enough so that those pans are already hot when the cook reaches for one. When orders start pouring in, the cooks won't have time to sit and wait for their pans to heat up.
Steaks and Chops Need to Be Cooked Quickly
And why is this important? If you're cooking a pork chop and the pan is too cool, it'll just sort of sit there while the pan slowly heats up. Next you'll start to see juices leak out, and those juices will then start to boil away as the pan gets hotter, thus causing your chop to steam rather than sear. When you flip it you'll see a sort of deathly gray color instead of the beautiful crispy brown crust it ought to have.
See, tender cuts of meat need to be cooked as quickly as possible so that they stay tender. A cold pan means the meat is going to spend more time over the heat, and it'll be tough as a result.
Sautéeing Veggies Requires a Hot Pan
It's the same with sautéeing veggies. Adding vegetables to a cool pan will cause them to steam rather than sauté, giving you drab, mushy, overcooked vegetables because they spent too much time over the heat. You want to cook veggies quickly so they stay crisp, flavorful and bright-colored. Which, again, means using a hot pan.
When Cooking Eggs, a Drop of Water Should Sizzle
A pan is hot enough to cook eggs when a drop of water will sizzle on it. You want that coating of fat to immediately start cooking the egg. If it's not hot enough, the egg will basically push the butter out of the way so the butter winds up on TOP of the egg rather than underneath it. Which means your egg is going to stick.
Generally speaking, you want to cook eggs in a hot pan with a little bit of fat in it — this goes for fried eggs as well as scrambled eggs. The fat can be butter or oil. Clarified butter is a good choice because you can get it hotter without it smoking.
But don't use too much. About 1/8 of an inch at the bottom of the pan is plenty. More and your eggs will be greasy; less and they'll stick.
Note, however, that once the egg has a moment to set, you should reduce the heat to low for the remainder of its cooking time so that it doesn't burn.
How Hot is Hot Enough?
This is the key question. Once upon a time I would advise people to heat their pan on high for 5 to 10 minutes. What I recently discovered, though, is that if you've got an electric stove, you will probably ruin your pan doing this. Sorry :(
So what I suggest is that you heat your pan over medium-high heat until a droplet of water will jump and skitter around on the surface of the pan. Make sure you do this test BEFORE you add oil to the pan, otherwise hot oil will spatter back up at you.
Eventually you will figure out the optimum preheating time for your stove and your preferred pan, and you won't need to use the drop of water anymore. I can tell when a pan is ready just by seeing the way the heat sort of shimmers off the surface.
Exceptions to the Rule
All right, we've seen these three examples of why it's important to let your pan heat up before you start cooking, but are there exceptions? As a matter of fact, yes.
When you're rendering the fat out of a piece of meat, such as skin-on duck breasts or chicken thighs, you actually DO want to start with a cold pan and bring it slowly up to heat. The same is true for bacon. If you're cooking bacon in a skillet, it should be cold to start off. (But note that there's a better way to cook bacon.)
Another exception is when you're caramelizing onions. Onions have a lot of water in them, and you want to slowly cook all that water out so that the sugars in the onions can then turn brown. And you need to go slowly because the sugars begin to caramelize at about 310°F, but water doesn't get any hotter than 212°F. So we start them in a cold pan and slowly heat up the pan and cook them gently over low heat. Cook them too fast and you'll brown the edges, but the water won't have a chance to cook off before the onions begin to burn. (By the way, a slow cooker is a great tool for caramelizing onions.)