Preheat oven to 400F. In a large pot of salted boiling water, cook the potatoes for 8 to 10 minutes. They should be slightly cooked but still firm. Drain and set aside.
In a medium-size skillet over medium-high heat melt and warm the butter and the olive oil. Add the onions and let them sweat for 5 minutes. add the bacon pieces to the skillet and cook for another 5 minutes until they have reached a light golden color.
Cut the raclette cheese into small cubes. Butter a gratin dish with butter. Layer half the potatoes and top with half the bacon-onion mixture and half of the cheese. Season with salt and pepper. Repeat the same with the remaining potatoes and mixture. Sprinkle the heavy cream over it. Place in the hot oven and cook for 15 to 20 minutes. Remove when the top is a light golden color and let it rest before serving.