PREHEAT oven to 350° F. Lightly grease 9-inch square baking dish.
COMBINE spinach soufflé, onion, and ricotta cheese. Fill shells with mixture.
Place shells in prepared baking dish. Cover with sauce.
BAKE for 25 to 30 minutes. Sprinkle mozzarella cheese over shells. Bake for an additional 5 minutes or until hot.