Place the chicken into a medium pot of water and bring to a rolling boil over medium-high heat. Boil for a few minutes, then turn to a medium-low heat and simmer for 30 to 45 minutes. Take the chicken out of the pot (leave the liquid for cooking the spaghetti) and let cool slightly. Pick out the meat to make 2 generous cups.
Preheat the oven to 350 degrees F.
Cook the spaghetti in the broth from the chicken until al dente. Do not overcook. Drain the spaghetti, reserving about 2 cups of the cooking liquid.
Add the spaghetti to a large bowl and combine with the chicken, mushroom soup, 1 1/2 cups of the Cheddar, the green and red peppers, seasoned salt, cayenne, onions and salt and pepper to taste. Mix in 1 cup of the reserved liquid at first, adding an additional cup if needed.
Place the mixture in a casserole pan and top with the remaining 1 cup Cheddar. Bake until bubbly, about 45 minutes. (If the cheese on top starts to get too cooked, cover the casserole with foil.)
The uncooked casserole can be covered and refrigerated for up to 2 days, or frozen for up to 6 months.