Meatball Sub Soup

Name: Meatball Sub Soup
Ingredients: 1 ( 14 - ounce ) bag store-bought Italian-style frozen meatballs; 2 tablespoons olive oil; 1 medium yellow onion , chopped (about 1 1/2 cups ); 2 cloves garlic , minced; 1 teaspoon kosher salt; 2 tablespoons tomato paste; 2 ( 15 - ounce ) cans crushed tomatoes with basil and oregano; 2 1/2 cups beef broth , plus more as needed; 1/4 cup grated Parmesan cheese; 1/4 teaspoon black pepper , plus more for garnish; 8 slices French baguette , cut 3/4 to 1-inch-thick; 1 cup ( 4 ounces ) shredded low-moisture mozzarella cheese