Slice potatoes into 1/4 inch thin slices using a sharp knife or mandolin.
Melt butter over low heat in a medium sauce pan. Saute diced onion until translucent. Stir in flour and seasonings. Cook over medium heat, stirring until the mixture is bubbly.
Gradually stir in milk and heat to boiling. Reduce heat and simmer for 3 minutes until thickened, stirring often.
Remove sauce pan from the heat and stir in the cheeses until melted completely.
Arrange potatoes in a greased 9x13 casserole dish in 3 layers. Top the first 2 layers of potatoes with 1/3 of the cheese sauce then add the final layer of potatoes and the remaining cheese sauce.
Smoke at 250°F for 90 minutes then increase the temperature to 375°F and cook for 30 more minutes until the cheese on top is bubbling and the potatoes are tender.
Allow potatoes to rest for 15-20 minutes lightly covered with foil. Garnish with parsley before serving.
