Choose a heavy sauté or fry pan for the best results. One with sloped sides is helpful because you can toss the foods as they cook. But a pan with straight sides works too.
Make sure the pan is large enough for the amount of food you’re sautéing or else the food will just steam.
Preheat the pan over medium-high heat. After a few minutes, hold your hand over the pan’s surface—if heat radiates from the pan, add oil and swirl it to coat the bottom.
Let the oil heat in the pan until it starts to “shimmer.” But don’t let it get so hot it starts to smoke—the oil will taste scorched.
Mushrooms: Add only enough mushrooms to cover the bottom of the pan in a single layer. Toss or stir the mushrooms until they brown and give off some of their moisture. If the pan seems dry or the mushrooms start to scorch, add more oil. Season with salt and pepper.
Onions: Add chopped or sliced onions and toss or stir to coat with oil. Cook the onions until they start to soften and turn translucent. If the pan seems dry or the onions start to scorch, add more oil. Season with salt and pepper.
Spinach: Add the spinach leaves to the pan – they will shrink down dramatically so don’t be afraid to mound the leaves. Use tongs to carefully turn the spinach leaves until they’re wilted. Season with salt and pepper.
Resist the urge to constantly toss or stir the mushrooms. Prolonged contact with the pan’s surface will help them brown.