The Best Baked Mostaccioli

Name: The Best Baked Mostaccioli
Ingredients: 1 tablespoon olive oil, plus more for greasing the baking dish; 1 small yellow onion, finely chopped; Kosher salt and freshly ground black pepper; 1/2 pound ground beef, 80/20; 1/2 pound sweet Italian sausage, casings removed (from about 3 sausage links); 4 cloves garlic, minced; 2 tablespoons tomato paste; 1/2 teaspoon dried oregano, plus 1/4 teaspoon more for the ricotta; Pinch crushed red pepper flakes (optional); One 28-ounce can crushed tomatoes, preferably San Marzano; 3 sprigs fresh parsley, plus more finely chopped parsley leaves for serving (about 1 tablespoon); 1 pound mostaccioli; One 15-ounce container whole-milk ricotta; 1 large egg; 1/4 cup plus 3 tablespoons finely grated Pecorino Romano; 8 ounces part-skim mozzarella, freshly shredded (about 2 cups)