Preheat the oven: Set to 375°F (190°C).
Lightly grease a 9x13-inch baking dish.
Shred the chicken: Remove skin if you like, and pull the meat into bite-sized pieces. Aim for 3–4 cups.
Boil the tortellini: Cook in salted boiling water for 2 minutes less than package directions. Drain well. Undercooking keeps it from getting mushy when baked.
Sauté aromatics: In a large skillet, warm olive oil over medium heat.
Add onion and cook 3–4 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Build the sauce: Pour in marinara, stir in cream, Italian seasoning, and red pepper flakes if using. Simmer 2–3 minutes.
Taste and season with salt and pepper.
Add greens: Stir in spinach and let it wilt, about 1 minute. If skipping spinach, move to the next step.
Combine: Add shredded chicken and drained tortellini to the skillet. Gently fold to coat everything evenly with sauce.
Layer in the dish: Transfer the mixture to the baking dish.
Sprinkle evenly with mozzarella and Parmesan.
Bake: Place in the oven for 18–22 minutes, until the cheese is melted and the edges are bubbling.
Broil for color (optional): Broil on high for 1–2 minutes to brown the top slightly. Watch carefully.
Rest and serve: Let the bake sit for 5 minutes to set. Garnish with chopped basil or parsley.
Serve hot with extra Parmesan on the table.