Start by bringing a large pot of salted water to a boil. Add 12 ounces of pasta (elbow or penne) and cook according to the package instructions until al dente. Drain the pasta and set it aside while you prepare the sauce.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent. Toss in the minced garlic and cook for another 30 seconds until fragrant.
Add the ground turkey to the skillet, breaking it apart with a spoon. Cook until the turkey is no longer pink and starts to brown, about 6-8 minutes. Season with salt, pepper, and 1 teaspoon Italian seasoning while it cooks.
Pour in the can of diced tomatoes (with juices) and the tomato sauce. Stir everything together, letting the mixture simmer for 5-7 minutes to blend the flavors and thicken slightly.
In a large mixing bowl or directly in the skillet (if large enough), combine the cooked pasta with the turkey tomato sauce mixture. Stir in half of the shredded mozzarella and cheddar cheese, mixing until evenly distributed.
Preheat your oven to 375°F (190°C). Transfer the pasta mixture into a greased 9x13-inch baking dish, spreading it out evenly. Sprinkle the remaining mozzarella and cheddar cheese on top for a golden, cheesy crust.
Bake the casserole in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and lightly browned on top. If you want an extra crispy top, you can broil it for an additional 2-3 minutes—just watch it closely to prevent burning.
Remove from the oven and let the casserole cool for a few minutes. Garnish with freshly chopped parsley if desired, then serve warm and enjoy!