Cheesy Meatball Casserole

Name: Cheesy Meatball Casserole
Ingredients: Cooking spray; 1/2 medium yellow onion; 3 cloves garlic; 1/4 cup fresh parsley leaves, plus more for garnish; 2 ounces Parmesan cheese, grated (about 1/2 cup); 1 1/2 pounds lean ground beef; 1/2 cup fine, dry breadcrumbs; 1 large egg; 2 teaspoons Italian seasoning; 1 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 8 ounces low-moisture mozzarella cheese, shredded (about 2 cups); 1 (about 24-ounce) jar marinara sauce, divided