Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.

Cook the spaghetti according to package instructions until al dente, about 8-10 minutes.

Melt butter in a skillet and sauté chopped onion, green bell pepper, and minced garlic for about 5 minutes.

In a mixing bowl, combine cream of mushroom soup, cream of chicken soup, chicken broth, and sour cream until smooth.

Stir in shredded chicken, sautéed vegetables, and seasonings until well combined.

Add cooked spaghetti to the mixture and toss until evenly coated.

Transfer the mixture to the greased baking dish and spread it evenly.

Sprinkle shredded cheddar and mozzarella cheese on top.

Bake for 25-30 minutes or until the cheese is bubbly and golden brown.

Let it cool for 5 minutes before serving.