Preheat the oven to 350°F.
Add the chicken to a stock pot. Cover with water and bring to a boil. Reduce the heat to medium and simmer until the chicken is cooked, about 25 minutes.
Remove the chicken from the water using tongs or a slotted spoon and set it aside on a plate to cool.
Remove 2 cups of broth from the pot and set aside.
Bring the remaining broth back to a boil and add the spaghetti. Cook it al dente.
Drain the spaghetti, discarding the cooking liquid. Set aside. With two forks (or your fingers), remove the chicken from the bones. Shred or cut the meat into bite-size chunks. Drain the pimientos. Then dice the vegetables.
Place the cooked spaghetti in a large bowl. Add the cream of mushroom soup, 2 cups of the cheddar cheese, onion, green pepper, and pimiento. Add the seasoned salt, black pepper, and cayenne pepper. Take it easy on the cayenne if you can’t handle the heat. But if you can, it sure adds some interest to the final dish.
Finally, add the chicken and broth. Stir together well, then taste to check the seasonings.
Pour the mixture into a large baking pan, and top with the remaining ½ cup cheddar cheese. Bake for 35 to 45 minutes, until bubbly.