Preheat oven to 350 degrees. Grease a 13x9 inch glass baking pan.Wash sweet potatoes and pat dry. Poke holes in the sweet potatoes with a fork, place them on a cookie sheet. Bake for 60 minutes or until they can be easily pierced with a knife. Cool the potatoes 20 minutes, then peel them and cut each potato into 1/4 inch slices. Set aside.Combine both grated cheeses in a medium bowl, set aside.Melt butter in a medium saucepan over medium heat. Whisk in the flour,salt and pepper. Cook and whisk until mixture is smooth and bubbly. Gradually whisk in half and half and cream. Cook and whisk about 7 minutes until mixture is slightly thickened. Add the dry ranch dressing packet, stir to combine. Stir in 3 cups of grated cheese (you should have one cup of grated cheese left to sprinkle over casserole before baking). Whisk until cheese has melted.Place the sweet potato slices , cheese sauce, spinach and cooked chicken in a large bowl, toss gently to combine. Spoon into prepared baking dish. Sprinkle the top of the casserole with the remaining one cup grated cheese. Bake 25 -30 minutes or until cheese has melted and casserole is heated through. Let stand about 10 minutes before serving.