Boil the potatoes in salted water until just tender, take care not to cook right through. Drain well and leave to cool. This is best done the day before. Meanwhile, melt half the butter in a fry pan and cook the onions and lardons until the onions are soft and both are beginning to brown a little. Tip in the wine, bring to a simmer, and reduce to nearly nothing. Stir in the cream off the heat.
Preheat the oven to 200C. Cut the potatoes into 1 cm cubes. Heat the remaining butter in a fry pan and saute until golden. Cut the cheese in half horizontally. Rub an ovenproof dish with the cut clove of garlic. Cover the base with half the potatoes, spoon over half the onion and bacon mixture and season. Top with half the Reblochon then repeat the layers, with the remaining Reblochon half, rind uppermost, on top. Bake for 15 minutes until browned and bubbling (stick it under the grill for five more minutes if you want it really crisp), then serve with a green salad, plenty of crusty baguette and a dry white wine.