Fill a large pot with approximately 16 cups of water and add 2 tablespoons of salt. Bring to a boil.
Once boiling add the spaghetti and cook for 10 minutes. Remove from heat and carefully pour into a strainer over the sink to remove the water.
Place the pot back on the stove and turn heat to medium. Add butter and let it melt.
Add the peppers and onions and cook for 2-3 minutes or until they begin to soften.
Add Cream of Mushroom soup, Cream of Chicken soup, diced tomatoes & chilies, shredded chicken, 2.5 cups of cheddar cheese, and cooked spaghetti to the peppers and onions and mix together.
Spray 13 x 9 inch casserole dish with cooking spray.
Pour the spaghetti mixture into the casserole dish and top with the remaining cheese.
Place in the oven and bake uncovered for 15-20 minutes or until hot and bubbly.
Remove from the oven and let stand for 10 minutes before serving.