Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Drain and set aside.
Step 2: Sauté Vegetables
In a skillet, heat olive oil over medium heat. Add onion and bell pepper. Cook for 3–4 minutes until softened.
Step 3: Mix the Sauce
In a large mixing bowl, combine cream of mushroom soup, cream of chicken soup, sour cream, garlic powder, onion powder, paprika, salt, and pepper. Stir well.
Step 4: Add Chicken and Veggies
Fold in the cooked chicken, sautéed vegetables, and drained diced tomatoes with chilies. Mix until evenly combined.
Step 5: Assemble the Casserole
In a greased 9×13-inch baking dish, spread the cooked spaghetti evenly. Pour the creamy chicken mixture over the pasta and mix gently.
Step 6: Add Cheese
Sprinkle 1 ½ cups of cheddar cheese on top. Save the remaining ½ cup for later.
Step 7: Bake
Bake in a preheated oven at 350°F (175°C) for 30 minutes. Remove, top with the remaining cheese, and bake for an additional 10 minutes until bubbly and golden.
