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. Add minced garlic and cook for an additional minute.
In a large bowl, combine the cooked spaghetti, shredded chicken, sautéed vegetables, cream of chicken soup, diced tomatoes, milk, paprika, chili powder, salt, black pepper, and cayenne pepper. Mix well until all ingredients are evenly coated.
Pour the mixture into the prepared baking dish and spread it out evenly. Top with shredded cheddar cheese.
Bake in the preheated oven for 25-30 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for 5 minutes. Garnish with chopped fresh parsley before serving.