For filling, combine the ¼ cup chopped stems, cheese, bread crumbs, parsley, walnuts, and butter, and then spoon 1 rounded tsp. of filling into each mushroom cap. Arrange half of the filled mushroom caps in a circle in the Casserole. Cook on high power 2½–3½ minutes or until heated through, giving the Casserole a half-turn once, if your microwave doesn’t have a turntable. Repeat with remaining filled mushrooms.