Cook pasta shells according to package directions. Drain and set aside. (Don’t worry if some shells tear – you won’t need them all.)
Meanwhile, cook the beef, onion, garlic and Italian seasoning in large, deep skillet set over medium heat until the beef is thoroughly cooked and any liquid has evaporated. Remove from heat. Stir in the ricotta, spinach, 1 cup of the mozzarella and pepper. Spoon about 2 tablespoons of the filling into each of about 32 shells.
Spread about a cup of the pasta sauce into each of 9x13-inch baking dish and a 9-inch glass pie plate (or similar sized baking dishes). Place 20 filled shells in the larger baking dish, and 10 to 12 in the pie plate. Drizzle the remaining pasta sauce over the shells, sprinkle with remaining mozzarella and Parmesan. (At this point the dish can be covered and refrigerated overnight or frozen for up to 4 months). Cover loosely with foil and bake in 350°F oven for 30 minutes. Uncover and bake for 5 minutes, until bubbly around the edges and golden on top.