Before cooking, either gently wipe mushrooms with a damp cloth or soft brush, or quickly rinse and immediately pat dry with paper towel. Never soak mushrooms because it speeds deterioration. The only trimming that maybe necessary is at the stem end if it looks dry.
Short cooking time yields a more delicately textured mushroom; longer cooking makes them denser and firmer.
Use a skillet large enough to cook mushrooms in a thin layer; otherwise they'll steam. Use about one tablespoon oil or butter per eight ounces of mushrooms. Heat the oil or butter before adding the mushrooms, so they'll brown quickly without losing excess moisture. Cook four to five minutes or until released liquid has evaporated.
Put mushrooms in a shallow baking pan, toss with oil and roast in a 450° F. oven until brown, about 20 minutes, stirring occasionally. Use about one tablespoon oil for each eight ounces of mushrooms.
Mushrooms microwave extremely well. Simply put eight ounces thickly sliced mushrooms in a microwave-proof bowl, cover and cook on High for two to three minutes, stirring once.
One pound of white mushrooms yields about five cups sliced or chopped, or about 2 cups sautéed. Four ounces of coarsely chopped Shiitake mushrooms yield about 1½ cups; 6 ounces of trimmed and coarsely chopped Portabellas yield about 2¼ cups.