Preheat oven to 400ºF.
For the béchamel, heat a small saucepot over medium heat with butter. Whisk in flour and milk then add in dry mustard. Add in hot sauce and bring to a boil; reduce heat to a simmer. Season with salt, pepper and nutmeg, and let thicken a few minutes until the sauce coats a spoon.
In a casserole dish, spoon a thin layer of béchamel and layer with sliced potatoes. Cover with a third of the shredded chicken, a third of the cheese and top with a third of the béchamel. Repeat layers twice and top with remaining béchamel and cheese.
Cover with foil and bake for 1 hour then remove foil and let brown for 30-45 minutes or until bubbly and golden brown.