Cut cleanly washed new or peeled old potatoes into slices.
Grease a medium size gratin dish and place a layer of potatoes, about 2 1/2 cm thick. Next, add a layer of onion rings. Season with salt and pepper. Next, repeat a layer of potatoes, this time arranged in a circle overlapping like roof tiles. A little salt and on top enough flakes of butter.
Cover the gratin dish with aluminum foil and bake in the oven at 130 °C for about an hour. Potatoes and onions steam in their own juice. The butter adds the rest.
As long as the potatoes still have a little “bite”, take the pan out of the stove and crumble a thick layer of Lancashire cheese on top. And again put it in the stove – this time without foil. After a few minutes at high temperature, the cheese will be au gratin and golden brown.
Try the casserole once just like that – or possibly also as a supplement to cold meat, roast beef, for example.