Cook spaghetti according to package directions until they're al dente. Drain, and set aside.
In a large mixing bowl, mix together the shredded chicken, diced green peppers, chopped onions, Rotel tomatoes, cream of mushroom soup, chicken broth, 1 1/2 cups of sharp cheddar cheese, minced garlic, salt, and pepper.
Add the cooked spaghetti to the chicken mixture and toss until well combined.
Transfer the mixture to a 9x13-inch baking dish and sprinkle the remaining shredded cheddar cheese over it.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving. Enjoy!