Brown ground beef and drain. Return to pan. Sauté onions and garlic in pan with cooked meat for approximately 3 minutes. Set aside. Boil salted water to cook the pasta shells according to package al dente style. Strain and set aside. In a large bowl, mix ricotta, Parmesan, and 2/3 package of shredded mozzarella. Add Italian seasoning, garlic pepper, eggs and milk and mix thoroughly. Mix in meat and spinach. In a large baking dish, pour enough sauce to cover the bottom of the pan lightly. Fill each shell with large rounded tbsp. of meat and cheese mixture, and place in pan. Be sure to closely place the shells together so they don't get hard while baking. Pour remaining sauce evenly over all of the shells. Sprinkle with remaining mozzarella cheese. Cover tightly with aluminum foil, and bake at 375 degrees for 35-45 minutes. Take foil off pan, and bake additional 10 minutes.
Ready in 1½ hour 25 min prep