Peel potatoes and cut them into 1-inch cubes so they cook faster. Add potatoes to cold water and season water with salt and pepper.
Cook potatoes until fork-tender and drain off the cooking liquid. Make sure to reserve some cooking water from cooking potatoes, don’t drain it all completely.
Add butter and milk and mash potatoes while it’s still hot. If needed, add a little more water, a little at a time, checking the thickness while mashing potatoes. Taste to see if more seasoning is needed.
Preheat a large saute pan over medium heat. Thinly slice leek and cabbage and set it aside.
Dice bacon into small pieces and add it to the pan. Add a little bit of canola oil if needed. Saute bacon until browned and fat is rendered.
Add leek and saute until softens. Then, add cabbage and season with some salt and pepper.
Cook over medium-low heat, stirring occasionally, until cabbage is fully cooked and soft. It will take about 10-15 minutes.
To combine potatoes and cabbage, simply add sauteed cabbage and bacon to the pot with mashed potatoes and carefully mix until combined.