PREHEAT broiler. Mix stuffing mix and hot water in medium bowl; cover. Let stand 5 min.
REMOVE stems from mushrooms; chop stems. Cook and stir mushroom caps in 3 Tbsp. butter in large frying pan until lightly browned. Arrange in shallow baking dish.
ADD stems and peppers to frying pan; cook and stir until tender. Stir into stuffing. Spoon into mushroom caps; drizzle with 2 Tbsp. melted butter.
BROIL 5 to 7 inches from heat 5 min. or until heated through.