Preheat and prep: Heat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
Cook the pasta: Boil the pasta in well-salted water until just shy of al dente (1–2 minutes less than package time).
Drain and set aside. Toss with a splash of olive oil to prevent sticking.
Sauté aromatics: Warm the olive oil in a large skillet over medium heat. Add the onion and cook 3–4 minutes until softened.
Stir in the bell pepper and cook 2–3 minutes more. Add garlic and cook 30 seconds until fragrant.
Brown the turkey: Add ground turkey to the skillet. Season with 1 teaspoon salt and 1/2 teaspoon pepper.
Cook, breaking it up with a spoon, until no longer pink and lightly browned, about 5–7 minutes.
Build the sauce: Stir in tomato paste, Italian seasoning, and red pepper flakes. Cook 1 minute. Pour in the marinara sauce and bring to a gentle simmer.
Fold in the spinach and let it wilt. Taste and adjust seasoning.
Combine with pasta: In a large bowl, mix the cooked pasta with the turkey sauce. Stir in 1/2 cup mozzarella and 1/4 cup Parmesan so it melts slightly and helps everything cling together.
Layer in the dish: Spread half the pasta mixture in the baking dish.
Dollop spoonfuls of ricotta across the layer. Sprinkle with a little mozzarella and Parmesan. Add the remaining pasta, then top with the rest of the mozzarella and Parmesan.
Bake: Cover loosely with foil and bake for 15 minutes.
Remove foil and bake another 10–12 minutes, until the cheese is melted and lightly golden at the edges.
Rest and serve: Let the bake rest 5–10 minutes so it sets and slices cleanly. Sprinkle with chopped basil or parsley. Serve hot with a green salad and extra Parmesan.