Bring a large pot of salted water to a boil. Add 8 ounces of rotini pasta and cook according to package instructions until al dente. Drain the pasta and set it aside.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add 1 pound of ground beef or turkey and cook until browned, breaking it apart with a spatula. Drain any excess fat if necessary. Add the chopped onion, minced garlic, and chopped bell pepper to the skillet. Cook for 4-5 minutes until the vegetables soften.
Stir in 1 can (15 ounces) of tomato sauce and 1 can (14.5 ounces) of diced tomatoes with their juice into the skillet. Add 1 teaspoon each of dried oregano, dried basil, and Italian seasoning. Season with salt and pepper to taste. Let the sauce simmer gently for 8-10 minutes to develop the flavors.
Add the cooked rotini pasta directly into the skillet with the sauce mixture. Stir well to coat the pasta evenly.
Preheat your oven to 375°F (190°C). Transfer half of the pasta mixture into a 9x13-inch baking dish. Sprinkle 1 cup of shredded mozzarella and half of the grated Parmesan over the pasta. Add the remaining pasta mixture on top, then finish with the remaining 1 cup mozzarella and ½ cup Parmesan cheese.
Place the casserole in the preheated oven and bake for 20-25 minutes, or until the cheese is melted, bubbly, and golden on top.
Let the casserole rest for a few minutes before serving. This allows the flavors to settle and makes it easier to slice. Serve warm and enjoy the irresistible combination of cheesy, saucy, and savory bites.