1. After cleaning, cut the different mushrooms as follows: For firm, regularly shaped mushrooms like cremini, portobellos, hedgehogs, puffballs, and shiitakes, trim the stems, if any, and thinly slice the caps, no more than ¼ inch thick and 1 inch long. For irregularly shaped mushrooms like hen of the woods and cauliflower, cut into ½-inch slices, then crosswise ½ inch thick. For soft mushrooms like chanterelles, yellowfoot, and black trumpets, cut strips ½ inch long and ½ inch across.
2. Melt the butter in a large, heavy skillet over medium heat until foaming. Stir in the mushrooms, a handful at a time, and cook each batch until the mushrooms wilt enough to show the bottom of the pan. Continue adding mushrooms until all are in the pan. Reduce the heat to low and cook, stirring occasionally, until the mushrooms seem to have given up all their liquid, 5 to 10 minutes, depending on the types of mushrooms. (If in doubt, let the mushrooms stand off the heat for 5 minutes to see if they give up any more liquid.) Turn the heat to high. Stir the mushrooms until the liquid is evaporated and the mushrooms begin to sizzle, about 3 minutes. Transfer the mushrooms-to a plate and let cool to room temperature.
3. Place the mushrooms in a food processor and use quick, on/off pulses until the mushrooms are finely chopped. Store, covered, for up to 1 week in the refrigerator or freeze for up to 3 months. To freeze cubes, place 1 tablespoon of the duxelles into each compartment of an ice-cube tray. Tamp down the duxelles and freeze with a piece of plastic wrap applied directly to the surface. When solid, transfer the cubes to a sealable plastic container or freezer bag.
Variations: Parmesan Duxelles: Stir 2 tablespoons grated Parmagiano-Reggiano or other good-quality Parmesan cheese into the duxelles after it has cooled to room temperature.
Creamy Duxelles: Stir 2 tablespoons heavy (or whipping) cream into the duxelles before removing from the heat.
Chunky Duxelles: Cool the sautéed mushrooms to room temperature. Transfer to a cutting board and coarsely chop. Do not use the food processor.