Preheat oven to 180°C
Line the thinly sliced U.S. potatoes into a lightly buttered baking dish
Melt butter in a small pot over low-medium heat
Add in onion and mushroom to cook, stirring occasionally until tender (about 5 minutes)
Stir in flour, then slowly add in the skimmed milk and stir till well combined. Bring to a slight simmer
Stir in the low-fat cheese, salt, black pepper and cayenne pepper
Pour mixture into prepared baking dish, then spread out to coat the thinly sliced potatoes
Bake in the oven uncovered for 45 minutes
After 45 minutes, cover with foil and continue to bake for another 10 minutes
Garnish with finely chopped English parsley