01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or a small amount of oil.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until just al dente, according to package directions. Drain well and set aside.
03 - Heat a large skillet over medium heat. Add ground turkey and cook, breaking up with a wooden spoon, until browned and cooked through, about 6-7 minutes.
04 - Add diced onion, garlic, and red bell pepper to the skillet with the turkey. Sauté for 4-5 minutes, stirring occasionally, until vegetables have softened.
05 - Stir in chopped spinach and cook for 1 minute until just wilted.
06 - Pour in marinara sauce, tomato paste, Italian seasoning, red pepper flakes (if using), salt, and black pepper. Stir well and let simmer for 3-4 minutes to allow flavors to meld.
07 - Add the cooked pasta to the skillet with the turkey sauce. Mix in half of the mozzarella and Parmesan cheeses, stirring until everything is evenly combined.
08 - Transfer the pasta mixture to the prepared baking dish, spreading it evenly. Sprinkle the remaining mozzarella and Parmesan cheese over the top.
09 - Bake uncovered for 20-25 minutes, or until the cheese is completely melted and bubbly, with lightly golden spots on top.
10 - Remove from oven and let the casserole rest for 5 minutes before serving. Garnish with fresh basil or parsley if desired.