Add the sliced potatoes, milk and corn flour to a saucepan, season, bring to the boil and turn down to simmer for 10 minutes. Stir occasionally to ensure all potato slices are cooked through.
Gently heat the butter in a skillet and sauté the garlic, mushrooms and onions until soft.
Tip the pan of sliced potatoes onto the contents of the skillet. Then arrange the potato slices on top of the onion and mushroom filling. Place into the oven for approximately 40 – 45 minutes until the potatoes have crisped on top.
Season with freshly ground salt and pepper, sprinkle over chopped parsley and serve.