Preheat oven to 400°F (200 degrees).
Wash potatoes and slice into thin slices, using a mandolin if you have one. Slice onion as thin as possible.
Layer potatoes and onions in little stacks. Place stacks upright in a greased casserole 20cm dish.
Melt butter & flour in a saucepan and cook 2-3 minutes. Add seasonings and milk. Whisk over medium heat until thick and bubbly. Remove from heat and stir in cheeses until melted.
Spoon cheese sauce over potatoes. Cover with foil (sprayed with cooking spray) and bake 60 minutes.
Remove foil and bake an additional 20-30 minutes or until lightly browned and potatoes are cooked. Cool 15 minutes before serving.
Serves: 4
Preparation: 15 mins
Cooking: 60 mins
1kg Albert Bartlett potatoes
1 onion
3 tbsp butter
3 tbsp flour
1 tsp dry mustard powder
240ml milk
250g grated cheddar cheese
125g gruyere cheese
salt & pepper to taste