In large bowl, combine ricotta, 1 cup mozzarella, 1/2 cup parmesan, egg and cooked spinach mixture. Mix until well combined. Stuff cooled shells with mixture. To assemble, spread 1 cup marinara sauce on bottom of pan. Top with half the sausage. Place shells in pan and cover with remaining marinara sauce and sausage. Top with remaining mozzarella and parmesan cheese. Cover pan with aluminum foil.