Preheat oven to 375 and lightly grease a 9X13 baking dish.
Bring a large pot of water to boil and cook pasta, cooking to just shy of recommended cooking time (i.e., if cooking time is 7 minutes, cook it for about 6). Drain well.
While pasta is cooking, remove sausage casings and cook sausage over medium heat, breaking up in pan. Add onion to pan when sausage is mostly browned and continue to cook for another 2-3 minutes or until sausage is fully cooked. Drain any excess grease.
In a small bowl, combine ricotta, 1 cup grated parmesan and egg. Mix until well combined.
Pour half of drained pasta in bottom of greased pan. Top with half of sausage, half of ricotta mixture, 1/3 of mozzarella and half of marinara sauce. Repeat with remaining pasta, sausage, ricotta mixture, 1/3 of mozzarella and sauce. Top with remaining parmesan and mozzarella.
Bake at 375 degrees, uncovered, for 45 minutes or until bubbling. Broil for a couple of minutes to brown the top.