Cook the tagliarini pasta according to package instructions. Drain and set aside.
In a large pot, cook the ground beef over medium heat until browned. Add the chopped onion and minced garlic, and cook until the onion is translucent.
Stir in the tomato sauce, diced tomatoes, dried oregano, and dried basil. Season with salt and pepper to taste. Let the mixture simmer for about 10 minutes.
In a large mixing bowl, combine the cooked pasta and the meat sauce. Mix well.
Transfer the pasta mixture to a baking dish. Sprinkle the shredded mozzarella and cheddar cheese evenly on top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving. Enjoy!
Notes:
This casserole can be made ahead and refrigerated. Just bake it when you're ready to eat.