HEAT oven to 400°F.
COMBINE rice, mushrooms, spaghetti sauce and 3/4 cup cheese.
STAND peppers in 2-qt. baking dish. Fill with rice mixture; cover.
BAKE 30 to 35 min. or until peppers are crisp-tender and filling is heated through. Top with remaining cheese; bake 5 min. or until melted.