01 - Preheat your oven to 400°F. Grease a 9x13 inch baking dish with olive oil or cooking spray.
02 - In a large mixing bowl, combine ground beef, Italian sausage, breadcrumbs, grated Parmesan, egg, minced garlic, chopped parsley, dried oregano, salt, and black pepper. Mix until just combined—avoid overworking the meat.
03 - Roll the mixture into 1-inch balls, approximately 24-30 meatballs. Place on a baking tray or plate.
04 - Heat a large skillet over medium-high heat with a splash of olive oil. Brown meatballs in batches for 5-6 minutes, turning to sear all sides. They need not be fully cooked through. Transfer to a plate and set aside.
05 - Bring a large pot of salted water to boil. Cook pasta until al dente, 2 minutes less than package directions. Drain well and set aside.
06 - In the same skillet, heat 2 tablespoons olive oil over medium heat. Sauté onion until softened, about 5 minutes. Add garlic and cook 1 minute until fragrant. Stir in crushed tomatoes, tomato paste, dried basil, chili flakes, sugar, salt, and pepper. Simmer 10 minutes until slightly thickened.
07 - Toss drained pasta with half the tomato sauce. Transfer to prepared baking dish. Arrange browned meatballs over pasta. Pour remaining sauce evenly over meatballs and pasta.
08 - Sprinkle shredded mozzarella and grated Parmesan evenly across the top.
09 - Bake for 25-30 minutes until cheese is golden and sauce is bubbling around the edges. Let rest 5 minutes before serving. Garnish with fresh basil if desired.