01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until al dente according to package directions. Drain well and set aside.
03 - Heat a large skillet over medium-high heat. Add ground turkey and cook for 6-7 minutes, breaking it up with a spoon, until browned and completely cooked through.
04 - Add diced onion, minced garlic, and red bell pepper to the skillet with the turkey. Reduce heat to medium and sauté for 4-5 minutes until vegetables are 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 simmer for 3-4 minutes to allow flavors to meld.
07 - Add cooked pasta to the skillet (or transfer everything to a large bowl). Mix in half of the mozzarella and half of the Parmesan cheese until evenly distributed.
08 - Transfer the pasta mixture to the prepared baking dish. Spread evenly and sprinkle with remaining mozzarella and Parmesan cheese.
09 - Bake uncovered for 20-25 minutes until cheese is melted, bubbly, and beginning to turn golden brown.
10 - Let the casserole rest for 5 minutes before serving. Garnish with fresh basil or parsley if desired.