Boil cubed potatoes in a medium sized pot together with 1 cup water. When water comes to a boil, season with salt. Reduce heat and allow it to cook for about 15 minutes or until potatoes are very soft. Turn off heat. Most of the water would have been absorbed but if there is excess liquid, drain it.
Add butter, mustard powder, and pepper. Pour in warm milk. Mash potatoes with a potato masher. Set aside.
Fry bacon in a large pan until almost crisp, about 5 minutes. Drain excess fat, leaving just a tablespoon. Stir in green onions followed immediately by cabbage until just tender, about 3 minutes. Turn off heat.
Add mashed potatoes and give it a quick gentle stir.