Choose potatoes (not new ones, which have a higher moisture content). Peel and grate them in a large bowl with the largest setting of a cheese grater. Add flour and a pinch of salt and mix well.
Scrape this mixture into an ovenproof baking dish greased with plenty of oil or lard. Brush the surface with oil or lard and bake in a preheated 400°F oven until there is a nice golden crunchy crust on top (at least 40 minutes).
Instead of a gratin, you can make croquettes with the same mixture. Just shape into little olive shaped balls, and fry in plenty of oil.