Peel and cut the Russet and Yukon Gold potatoes into chunks, place in a pot with salted water, and boil for 15-20 minutes until fork-tender. Drain and return to warm pot.

In a skillet, melt the unsalted butter, add chopped onions, and sauté for 5-7 minutes until translucent. Stir in shredded cabbage and cook for another 7-8 minutes.

Mash the warm potatoes in the pot, mix in the heavy cream and remaining butter until creamy. Fold in the sautéed cabbage and onions, and season with salt and pepper.

Transfer to a serving bowl, top with remaining bacon, and garnish with butter if desired. Serve warm.