01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or a thin layer of olive oil.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until just al dente, approximately 1-2 minutes less than package directions. Drain thoroughly 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 completely 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 until vegetables are softened and fragrant.
05 - Stir in chopped spinach and cook for 1 minute until just wilted.
06 - Add marinara sauce, tomato paste, Italian seasoning, red pepper flakes, salt, and black pepper. Stir to combine and let simmer for 3-4 minutes to allow flavors to meld.
07 - Add cooked pasta to the skillet and toss to coat evenly. Mix in half of the mozzarella and half of the Parmesan cheese until incorporated.
08 - Transfer the pasta mixture to the prepared baking dish, spreading evenly. Sprinkle remaining mozzarella and Parmesan cheese over the top in an even layer.
09 - Bake uncovered for 20-25 minutes until cheese is melted, bubbly, and beginning to turn golden brown around the edges.
10 - Remove from oven and let rest for 5 minutes to allow the casserole to set. Garnish with fresh basil or parsley before serving.