1. Pre-heat the oven to 190°C.
2. Cook the potatoes in boiling salted water until just tender to the point of a knife. Drain. Once the potatoes are cool enough to handle, peel and slice into half centimetre thick discs.
3. Heat half the butter and half the oil in a large frying pan. Add the potatoes and fry until golden. Remove from the heat. Season the potatoes with salt and pepper and put in either a large, shallow, buttered ovenproof dish or divide between six individual gratin dishes.
4. Heat the rest of the butter and oil in the same frying pan and cook the lardons over a medium high heat until they are crisp and coloured. Turn down the heat and add the onions. Continue cooking until the onions are softened. Add the garlic and continue to cook for another 2-3 minutes, until the garlic is fragrant. Remove from heat and stir in the single cream.
5. Pour the lardon and onion mixture over the potatoes then cover with slices of Gruyère. Bake for 15-20 minutes, or until the cheese is melted and bubbling and browned around the edges.
6. Remove from the oven, scatter over the snipped chives and serve.