Preheat the oven to 425°F / 220°C. Peel and thickly slice 750g of potatoes. Boil in salted water for 8-10 minutes, until just tender, then drain.
Fry 1 finely chopped onion in 25g of butter and 1 tablespoon of olive oil for 5 minutes. Add 6 slices of smoked bacon, snipped into pieces, and cook for 5 minutes more, until lightly colored.
Cut 250g of Reblochon into chunks, rind and all. Layer half the potatoes in a buttered 1½-quart / 1.5-liter baking dish and scatter over half the onion, bacon, and cheese. Season lightly and repeat.
Pour 150ml of light cream evenly over the top. Bake for 10-12 minutes, until golden, then rest for 5 minutes.
