Boil the potatoes in well-salted water until just tender to a fork, but not cooked right through. Drain well and leave to cool enough to handle.
Meanwhile, melt half the butter in a frying pan and sauté the onions and bacon until the onions are soft and both are beginning to brown. Add the garlic and cook another minute.
Tip in the wine, bring to a simmer, and reduce to at least half.
Stir in the cream off the heat.
Preheat the oven to 200C/400F/gas mark six. Cut the potatoes into smallish cubes (roughly 1/2-1″). Heat the remaining butter in a frying pan and sauté them until golden (optional).
Pour the onion/bacon/cream mixture over potatoes, mix. Top with the Reblochon, cut in half (the long way – like a burger bun), rind facing up.
Bake for 15 minutes until browned and bubbling (stick it under the grill for five more minutes if you want it really crisp).
Notes
Serve with a green salad and a glass of dry white wine.