Meanwhile, melt the butter and oil in a skillet on medium heat and add the chopped onions. Cook for five minutes stirring occasionally. Add the bacon and cook another five minutes. Sprinkle with thyme and remove from heat.
In a buttered baking dish, place a layer of sliced potatoes using half of the potatoes. Layer with half of the bacon and onion mixture and then repeat the two layers. Pour the wine evenly over this.
Cut the Reblochon into two discs, and place the slices rind side down on top of the potatoes and bacon.
Bake at 400°F for 20 minutes - the cheese should be barely golden when you take it out of the oven.
Allow to cool just a few minutes. Serve hot.
Makes 4 hearty servings.