Preheat oven to 400 degrees.Advertisement
In a medium bowl, combine ricotta, mozzarella, 1/2 cup Parmesan, egg, sugar and parsley; season with 1/2 teaspoon each salt and pepper.
Spread half the marinara in the bottom of a 9-by-13 inch baking dish. Fill each shell with ricotta mixture and arrange in an even layer in dish. Top with remaining 2 cups marinara. Cover with foil and bake until sauce is bubbling and shells are tender, about 40 minutes.
Sprinkle remaining Parmesan over top, return to oven, and bake until cheese is browned in spots, about 10 minutes.