Once melted and combined, remove sauce from heat and stir in 2 cups of sharp cheddar, a small handful at a time, stirring well between additions to incorporate.
Immediately add to cooked spaghetti mixture.
Gently fold in cooked and cubed chicken.
Pour into a greased casserole dish. Cover with remaining 1 cup grated sharp cheddar cheese and bake at 350 degrees for about 30 minutes or until hot and bubbly.