Peel and thinly slice the potatoes. Set to one side.
In a saucepan, melt the marg then add the flour, stirring to soak up the marg. If the roux is quite wet, add a little more flour until it looks like the photo below.
Slowly add the milk, stirring continuously after each addition to ensure no lumps. After you’ve added about half the milk, turn the heat up and bring it to a boil. As the sauce thickens, whisk with a balloon whisk to stop it clumping.
When the sauce has thickened, add the rest of the liquid until you have a sauce of pouring consistency. Let it come up to the boil again and stir while it bubbles for a few minutes. Turn the heat down and add the cheese. Add nutmeg and seasoning to taste.
Lightly grease a baking dish, then layer up your potatoes into the dish.
Pour the cheese sauce over the top, cover with foil and bake at 160 C for 1 hour. Remove the foil and turn up the heat to 200 C and let the bake brown for about 20 mins. Check the potatoes are cooked through before serving.
