HEAT broiler.
ADD hot water to stuffing mix in medium bowl; stir just until moistened. Let stand 5 min.
MEANWHILE, remove stems from mushrooms; chop stems. Cook and stir mushroom caps in 3 Tbsp. butter in large skillet 5 min. or until lightly browned. Place, rounded-sides down, in shallow baking dish.
ADD stems and peppers to skillet; cook and stir 5 min. or until tender. Add to stuffing; mix well. Spoon into mushroom caps; drizzle with melted butter.
BROIL, 6 inches from heat, 5 min. or until heated through.
