Preheat the oven to 375 degrees F. Bring a large pot of salted water to a boil. Grease a 10-inch cast-iron pan with butter and set aside.
Add the sliced potatoes to the boiling water and cook until just tender, about 10 minutes. Carefully drain and set aside.
Add the bacon to a large cast-iron pan over medium heat and allow the bacon to begin browning and rendering fat, about 5 minutes. Add the sliced red onion and continue to cook to soften the onions, for another 5 minutes. Add the Swiss chard, season with salt to taste and cook until wilted, about 5 minutes.
Layer the greased cast-iron pan with a third of the potatoes, half of the gruyere, then half of the bacon-onion-chard mixture. Repeat. Finally, top with the remaining potatoes and the cream. Decorate the top with the reblochon cheese in an even layer covering the potatoes. Bake until golden brown, about 20 minutes.