Spaghetti and Meatballs

Name: Spaghetti and Meatballs
Ingredients: 2 28-ounce cans whole peeled tomatoes in juice, drained, juice reserved, tomatoes finely chopped; 1/2 cup (1 stick) unsalted butter; 2 medium onions, peeled, halved through root end; 1/2 teaspoon (or more) salt; 1 cup fresh breadcrumbs made from crustless French or country-style bread; 1/3 cup whole milk; 8 ounces ground beef (15% fat); 8 ounces ground pork; 1 cup finely ground (not grated) Parmesan cheese; 1/3 cup finely chopped Italian parsley; 1 teaspoon salt; 1/4 teaspoon freshly ground black pepper; 2 large eggs; 2 large garlic cloves, pressed; 1 pound spaghetti; Freshly grated Parmesan cheese (for serving)