In a really large skillet or cooking pot, sauté bell pepper, onion, and garlic in olive oil. Add chicken chunks and sauté until evenly browned, about 5-8 minutes. It’s OK if chicken is still pink in the middle. Add 1 can of chicken broth and bring to a boil.  Add both soups, Ro-Tel tomatoes, and cheese, mixing well.  Bring back to a boil, then reduce heat and simmer for about 10 minutes until chicken is cooked through and cheese is totally melted.

Meanwhile, cook the spaghetti until just barely "al dente" - it will continue to cook through in the oven. Drain, rinse, and set aside. In the same pot you cooked the spaghetti, combine the chicken mixture, cooked spaghetti, drained mushrooms if using, and salt and pepper to taste. Depending on how creamy you like this dish, you may want to add up to an entire can of extra chicken broth also (I always do) because the spaghetti will absorb some of the liquid as it continues to cook.  Pour into a large greased baking dish. Bake at 375 degrees for 30 minutes or until hot and bubbly.