Preheat oven to 375

Slice potatoes and onion to 1/8 in thick and arrange in greased 8x12 pan in 3 rows

Melt butter over medium heat in skillet, whisk in flour, milk, and seasoning, stirring constantly to make a roux

Gradually add cheddar cheese and heat until gravy consistency

Pour cheese sauce over potatoes/onions in baking pan

Cook at 375 for 45 mins

Pull from oven and sprinkle with Gruyère and Parmesan cheese

Return to oven and cook 45 mins at 375

Garnish with parsley