Spaghetti and Meatballs

Name: Spaghetti and Meatballs
Ingredients: 1 small yellow onion; 4 cloves garlic, divided; 1 small bunch fresh parsley, plus more for serving; 1 ounce Parmesan cheese, finely grated (about 1/2 firmly packed cup grated on a Microplane or 1/3 cup store-bought), divided; 8 ounces ground beef; 8 ounces ground pork; 2/3 cup panko or fresh breadcrumbs; 1/4 cup milk; 1 large egg; 1 1/2 teaspoons kosher salt, divided, plus more for the pasta water; 1/4 teaspoon red pepper flakes (optional); 1 (28-ounce) can crushed tomatoes; 1 pound dried spaghetti