Parcook the potatoes: Cut the potatoes into 1/4-inch (6 mm) slices.
Place in a pot of cold salted water, bring to a gentle boil, and cook for 8–10 minutes until just tender but not falling apart. Drain and let steam-dry.
Brown the bacon: Heat a large skillet over medium heat. Add the bacon and cook until golden and crisp at the edges, 6–8 minutes.
Transfer to a plate, leaving some fat in the pan.
Cook the onions: Add 1 tablespoon butter to the pan if needed. Add the onions and a pinch of salt. Cook over medium heat, stirring occasionally, until soft and lightly caramelized, about 10 minutes.
Sauté the mushrooms: Push onions to the side and add the olive oil plus mushrooms.
Cook without stirring for 2–3 minutes to brown, then stir and continue cooking until they release moisture and it evaporates. Season with salt, pepper, and thyme if using.
Add garlic and deglaze: Stir in the garlic for 30 seconds. Pour in the white wine, scraping up browned bits.
Simmer until almost all the liquid has reduced, 2–3 minutes. Return the bacon to the pan and mix.
Preheat and prep dish: Heat the oven to 375°F (190°C). Butter a medium baking dish (about 9x9 inches or similar).
Layer everything: Lay half the potatoes in the dish.
Season lightly with salt and pepper. Spoon over half the bacon-onion-mushroom mixture. Repeat with remaining potatoes and mixture.
Add cream: Pour the heavy cream evenly over the layers.
It should settle into the gaps without drowning the dish.
Top with Reblochon: Slice the Reblochon horizontally through the middle to make two thinner wheels. Place them rind-side up on top of the potatoes, covering as much surface as possible. Cut into chunks if needed to fit.
Bake: Bake for 20–25 minutes until the cheese is melted and bubbling.
Switch to broil for 1–3 minutes to brown the top lightly, if you like.
Rest and serve: Let the tartiflette rest 10 minutes so it sets slightly. Serve warm with a simple green salad and sharp vinaigrette.