Preheat your oven to 375°F (190°C).
Cook the spaghetti according to package instructions until al dente.
In a large bowl, combine the cooked spaghetti, shredded chicken, ricotta cheese, mozzarella cheese, Parmesan cheese, cream of mushroom soup, marinara sauce, bell pepper, onion, garlic, and Italian seasoning.
Season the mixture with salt and pepper to taste.
Transfer the mixture into a greased 9x13-inch baking dish, spreading it evenly.
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 15-20 minutes, until the top is bubbly and golden.
Let the casserole cool for a few minutes before serving.