Melt the butter in a large frying or cast iron pan over a medium heat and stir in the bacon lardons or pancetta. Fry for 5-8 mins until starting to crisp up and the fat has rendered. Season with ¼ tsp freshly ground black pepper and cook for about 1 min more to toast the pepper, then stir in the onions. Season with a pinch of salt, then continue to cook for another 5-8 mins until the onions are soft but not too deeply coloured.
Meanwhile, peel the potatoes and cut into slices slightly thicker than the width of a £1 coin. When the onions are ready, tip the potatoes into the pan and cook everything for 10 mins, tossing regularly until the potatoes are nearly cooked through and have a taken on a bit of colour.
Carefully pour in the wine and bubble for 5 mins, still tossing the pan occasionally, until the wine has reduced completely and the potatoes are tender when pierced with the tip of a knife. Remove from the heat, spoon over the crème fraîche and grate over a small amount of nutmeg. Stir everything together so the potatoes are completely coated in the crème fraîche.
Cut the cheese in half through the equator so you have two thinner rounds. Chop one of the rounds into small chunks, then stir these through the potato mixture. Sprinkle a little salt into a medium gratin dish, then rub the cut side of the garlic clove all over the inside of the dish. Discard the garlic. Scrape the potato mixture into the dish, then nestle the other cheese round, rind-side up, onto the top of the mixture. At this stage, the tartiflette will keep up to a day ahead. Cover and chill until needed, then remove from the fridge 1 hr before cooking.
Heat the oven to 220C/200C fan/gas 8. Bake the tartiflette for 20-25 mins until the cheese is oozing and golden, and the mixture is bubbling at the edge. Leave to stand for 5 mins, then serve with cornichons and a bitter leaf salad.
