Step 1: Cook the Spaghetti: Bring a large pot of salted water to a boil and cook the spaghetti until slightly under al dente, about 2 minutes less than directed. Drain and set aside.

Step 2: Sauté Vegetables: In a skillet over medium heat, add olive oil and sauté chopped onions and bell peppers for 4-5 minutes until tender. Add minced garlic and cook for an additional minute.

Step 3: Simmer the Chicken Mixture: Add shredded chicken, cream of chicken soup, undrained Italian diced tomatoes, chicken broth, and Italian seasoning to the skillet. Stir and simmer for about 5 minutes.

Step 4: Combine with Spaghetti: Add cooked spaghetti and half of the shredded cheeses to the mixture, tossing gently until well coated.

Step 5: Transfer to Casserole Dish: Transfer the mixture to a greased casserole dish, spread evenly, and top with remaining cheeses.

Step 6: Bake to Perfection: Preheat oven to 350°F (175°C) and bake uncovered for about 30 minutes until bubbly and golden.