If desired, freeze leftovers in single servings--spaghetti, three meatballs, and sauce.
Combine ground round, 1 Tbsp. onion, and next 2 ingredients in a large bowl just until blended. Gently shape meat mixture into 18 (1 1/2-inch) balls.
Place a lightly greased rack in an aluminum foil-lined broiler pan. Arrange meatballs on rack.
Bake at 350° for 15 minutes or until browned. (Centers will be slightly pink.)
Prepare pasta according to package directions. Keep warm.
Cook chili powder in hot oil in a Dutch oven over medium heat, stirring constantly, 2 minutes. Stir in tomatoes and next 3 ingredients. Gently stir meatballs into tomato mixture. Bring to a boil; cover, reduce heat, and simmer, stirring occasionally, 10 minutes. Gently stir in beans, and cook 3 more minutes. Serve immediately with spaghetti and desired toppings.