In 10-inch skillet, melt remaining 2 tablespoons butter over medium heat. Add onion, salt and pepper. Cook 3 to 4 minutes or until softened and browned. Add mushrooms; cook 5 to 6 minutes or until mushrooms release juices, then turn brown. Add spinach; stir 1 to 2 minutes or until wilted. Pour over salmon in baking dish. Top with bread crumb mixture.