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1. In a large skillet, heat the olive oil over medium heat. Once hot, add the chopped onion and sauté for about 3-4 minutes until it becomes translucent.
2. Add the minced garlic to the skillet and cook for an additional 1-2 minutes, stirring frequently to prevent burning.
3. Next, add the ground beef to the skillet. Use a wooden spoon to break it up into smaller pieces. Cook until the beef is browned and no longer pink, about 5-7 minutes. Make sure to drain any excess fat from the skillet.
4. Once the beef is cooked, stir in the diced tomatoes (with their juice), tomato sauce, oregano, basil, salt, and black pepper. Bring the mixture to a simmer and let it cook for about 10 minutes, allowing the flavors to meld together.
5. While the beef mixture is simmering, bring a large pot of salted water to a boil. Add the pasta and cook according to the package instructions until al dente. This usually takes about 8-10 minutes.
6. Once the pasta is cooked, drain it in a colander and set it aside. Make sure to rinse it briefly with cold water to stop the cooking process and prevent it from becoming mushy.
7. In a large mixing bowl, combine the cooked pasta with the beef mixture. Stir well to ensure that the pasta is evenly coated with the sauce.
8. Add the sour cream and milk to the mixture, stirring until everything is well combined. This will add creaminess to the casserole.
9. Next, fold in 1 cup of the shredded cheddar cheese and 1/2 cup of the mozzarella cheese. This will give the casserole a cheesy flavor throughout.
10. Preheat your oven to 350°F (175°C).
11. Grease a 9×13-inch baking dish with cooking spray or a little olive oil to prevent sticking.
12. Pour the beef and pasta mixture into the prepared baking dish, spreading it out evenly.
13. Sprinkle the remaining cheddar cheese and mozzarella cheese on top of the casserole. Then, add the grated Parmesan cheese for an extra layer of flavor.
14. Cover the baking dish with aluminum foil to prevent the cheese from browning too quickly. Bake in the preheated oven for 25 minutes.
15. After 25 minutes, remove the foil and continue baking for an additional 15-20 minutes, or until the cheese is bubbly and golden brown.
16. Once done, remove the casserole from the oven and let it sit for about 5-10 minutes before serving. This will help it set and make it easier to serve.
17. Using a spatula, cut the casserole into squares and serve it warm. Garnish each serving with a sprinkle of fresh parsley for a pop of color and freshness.

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