2 kg floury potatoes , such as Maris Piper or Idaho
30 g unsalted butter
2 tablespoons plain flour
500 ml semi-skimmed milk
300 ml double cream
75 g Cheddar cheese
75 g Gruyère cheese
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Peel and cut the potatoes into 3mm-thick slices, then cook in a large pan of boiling salted water until just tender. Drain well, then transfer to a large baking dish.
Melt the butter in a large non-stick pan on a medium heat, stir in the flour, followed slowly by the milk and cream, then simmer gently until thickened, stirring continuously.
Remove from the heat, grate in and stir through most of the cheese, then season to perfection with sea salt and black pepper.
Pour the sauce over the potatoes, grate over the remaining cheese, then bake for 45 minutes, or until golden and bubbling.