While the potatoes are boiling, cook the bacon in a skillet over medium high heat. When the bacon has browned and is slightly crisp, remove to a paper towel lined plate. Reserve about a tablespoon of the bacon grease and drain the rest. Leave the rest of the drippings in the skillet. Add the onions to the hot skillet with the bacon grease and saute over medium high heat for 1-2 minutes, until they are lightly browned. Add the cabbage and toss lightly while it sautes and wilts, about 5 minutes. Season generously with salt and pepper as it cooks.
Drain and place the potatoes back in the hot pot. Place the pot back over the still-warm burner (or if your stove is gas, set the heat to the lowest setting). Mash the potatoes over very low heat and add the milk, 1/2 - 1 teaspoon salt and pepper. Fold in the cooked and crumbed bacon, cabbage, and onions. Transfer to a serving bowl, form a small well in the center and fill with the butter. Serve hot. Enjoy!