Step 1 of 6

Heat the oven and par-boil the potatoes

Heat the oven to 375°F. Bring a large pan of salted water to a boil, slide in the sliced potatoes, and par-boil for 5 to 8 minutes until bendy but not fully soft — a knife should still meet a little resistance. Drain well and leave them to steam-dry in the colander.

— Par-boiling is what lets the gratin bake in well under an hour; raw slices would need almost twice as long.