1. Trim and discard tough or discolored stem ends from mushrooms and any bruised spots or blemishes. If using chanterelles, submerge mushrooms in cool water and gently agitate with your hands to loosen any dirt. Drain, rinse thoroughly under running water, and gently pat dry with a towel. If using oyster or common mushrooms, rinse well and drain. Cut mushrooms into bite-size pieces or quarter lengthwise.
2. In a 10- to 12-inch frying pan over high heat, melt 2 tablespoons butter. Add mushrooms and stir often until liquid is evaporated and mushrooms are browned, 6 to 10 minutes. Add salt and pepper to taste.
3. In a 5- to 6-quart pan over high heat, bring 2 1/2 to 3 quarts water to a boil. Trim off stem ends from beans, and remove any strings. Add beans to water and cook until barely tender to bite, 5 to 8 minutes. Drain. Return drained beans to pan with remaining tablespoon butter; mix to coat and add salt and pepper to taste. Pour green beans onto a platter and spoon mushrooms on top.