Slice the potatoes thinly, put them in a bowl, cover them and heat in microwave for 3 mins.
In a bowl beat the egg yolk (not the white) and the cream or milk then add to the frying pan with the onions.
Layer potatoes in casserole dish and pour cream mixture over. Sprinkle grated cheese on top and put in the oven on a low heat. Keep checking every quarter of an hour or so to see how it looks, bump up the heat if you're hungry but keep an eye on it so it doesn't burn on top.