Preheat the oven to 200°C.
Boil whole potatoes in a large pan for 10 minutes.
Fry butter, onion, and thyme in a pan for 8 minutes until soft.
Drain potatoes, peel with a knife, and cut into 5mm thick slices.
Rub an ovenproof dish with cut garlic and smear with butter.
Layer onions, six bacon slices, potato circles, and cheese in the dish.
Dot with half the remaining butter and season with salt and pepper.
Bake for 45 minutes until potatoes are crisp and cheese is melted.
Fry Remaining chopped bacon in butter until crisp and browned.
Whisk mustard, vinegar, and yolk, then slowly emulsify with vegetable oil.
Combine the oil mixture with the hot bacon fat to finish the dressing.
Toss salad leaves with crisp bacon and the prepared dressing.
Brush the tartiflette with butter and serve with the salad.