“I switched from nonstick to stainless pans, and now when I sauté onion and garlic they burn or turn to mush. What am I doing wrong?”
When you used your nonstick pans, you may have used little or no oil to cook garlic and onion over medium high heat. If you try to do that in pans without a nonstick surface, the vegetables will scorch. You need a thin coating of oil in the pan, and you can’t cook the garlic and onion over very high heat for more than a couple of seconds.
As a rule of thumb, a 12-inch skillet is nicely, but not excessively coated with 1 tablespoon of oil.