Cook the spaghetti according to the package directions until al dente. Drain and set aside.
In a large mixing bowl, combine shredded chicken, diced green peppers, chopped onions, Rotel tomatoes, cream of mushroom soup, chicken broth, 1½ cups of grated sharp cheddar cheese, minced garlic, salt, and pepper. Mix thoroughly.
Add the cooked spaghetti to the chicken mixture and toss gently until well coated.
Transfer the mixture to a greased 9×13-inch baking dish and sprinkle the remaining cheddar cheese over the top.
Bake in the preheated oven for 25-30 minutes until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving.
Notes
For more flavor, consider adding a dash of hot sauce or chili flakes.
Leftovers can be stored in an airtight container for up to three days.