What cooking skills are important to know?

In the last year or two I’ve gotten really interested in cooking and started teaching myself new recipes. It’s becoming clear that there are better and worse ways to do things. If you want to saute a chicken breast, you can cook it one way and it will come out moist and tender. Cook it another way and it will be tough and dry.

What are some everyday techniques that you use, to be a better cook? Stuff like “don’t crowd the mushrooms.”