1. Crumble ground beef and sausage into a large bowl. Press garlic through a garlic press. Add to mixture with spinach (do not squeeze dry), bread crumbs, 1/2 cup of the Parmesan, eggs, salt and pepper. Mix and shape by rounded tablespoonfuls into 5 dozen meatballs. Arrange meatballs on a cookie sheet.
2. Bring tomato sauce to a simmer, stirring occasionally, in a large Dutch oven. Carefully drop meatballs, 1 at a time, into sauce. Cover and simmer 30 minutes, swirling the pan occasionally to prevent meatballs from sticking.
3. Meanwhile, heat oven to 375 degrees F. Cook pasta according to package directions just until al dente, about 8 minutes. Drain. Transfer pasta to a shallow 4-quart baking dish. Add sauce and meatballs, then stir all ingredients to combine. Cover dish loosely with foil and bake 20 minutes.
4. Combine ricotta and remaining 1/2 cup Parmesan in a bowl.
5. Uncover dish and sprinkle with mozzarella; stir to combine. Spoon rounded tablespoonfuls of ricotta mixture over pasta. Cover loosely; bake 15 minutes more. Sprinkle with parsley. Makes 12 servings.