Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or olive oil.

In a large skillet, heat olive oil over medium heat. Add diced onion and bell pepper, sautéing until softened, about 5 minutes.

In a large mixing bowl, combine the cooked spaghetti, shredded chicken, cream of chicken soup, sour cream, cheddar cheese, diced tomatoes, garlic powder, paprika, cayenne pepper, salt, and pepper. Mix well until all ingredients are evenly coated.

Stir in the sautéed onion and bell pepper mixture into the spaghetti mixture.

Pour the mixture into the prepared baking dish and spread it evenly.

Sprinkle the grated Parmesan cheese over the top.

Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the top is golden brown.

Remove from the oven and let it cool for about 5 minutes before serving. Garnish with chopped fresh parsley if desired.