HEAT oven to 350°F.
MELT butter in small skillet on medium heat. Add onions and garlic; cook and stir 3 min. or until onions are crisp-tender. Place in medium bowl. Add spinach, ricotta, mozzarella, Parmesan, eggs, lemon juice and pepper; mix well. Spoon into manicotti shells.
PLACE in 13x9-inch baking dish sprayed with cooking spray; cover.
BAKE 40 min. or until heated through. About 5 min. before manicotti is done, heat the spaghetti sauce.
SERVE manicotti topped with sauce.