Preheat the oven to 425F (220C).
Place the bacon and the onions in a large, oven proof skillet and cook over medium-high heat until the bacon and onions begin to turn dark golden, about 8 minutes, stirring constantly. Drain away all but 1 tablespoon of the bacon fat, then add the butter and stir. Add the potatoes and stir so that all the ingredients are thoroughly mixed in the skillet. Stir and shake the pan until the potatoes begin to brown, then reduce the heat to medium, cover, and cook until the potatoes are tender, 15 to 20 minutes, stirring and shaking the pan regularly.
While the potatoes are cooking, cut the Reblochon crosswise in half. Slice one half through the center horizontally. Cut the other half into ½-inch (1.25cm) cubes. If using Raclette, cut half of it into slices, and the remaining cheese into cubes. Return to the refrigerator.
When the potatoes are tender, remove the pan from the heat and fold in the chilled cubes of Reblochon. Place the remaining Reblochon, cut side down, atop the potatoes.
Bake until the Reblochon is completely melted and golden on top, about 20 minutes. If the cheese isn’t golden, turn on the broiler and broil the cheese until it turns golden, which will take 5 to 6 minutes.
Remove from the oven, strew with some parsley sprigs, and serve immediately.