Chop leeks, then wash see (
see Tips). Heat butter in a 10-inch heavy skillet over medium-high heat until foam subsides, then sauté leeks with 1/2 tsp salt until tender, 8 to 10 minutes. Add mushrooms and soaking liquid and boil until most of liquid has evaporated. Stir in soy sauce and remove from heat. Season with salt and pepper.