Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.

Cook spaghetti noodles according to package instructions until al dente. Drain and set aside.

Heat olive oil in a large skillet over medium heat. Sauté onions until translucent, 3-4 minutes, then add garlic and cook 1 minute.

Add ground beef to the skillet and cook until browned. Drain excess fat.

Stir in tomato sauce, diced tomatoes, Italian seasoning, red pepper flakes, salt, and pepper. Simmer 5-7 minutes.

Combine cooked spaghetti with the meat sauce, tossing to coat evenly.

Layer half of the spaghetti mixture in the baking dish, sprinkle half of the cheeses. Top with remaining spaghetti and cheeses.

Cover with foil and bake 20 minutes. Remove foil and bake 10-15 minutes until cheese is bubbly.

Let rest 5 minutes. Garnish with parsley and serve.