We add celeriac, sometimes called celery root, since it pairs nicely with potatoes and adds another element of flavor.
For the shopping list
head green cabbage, chopped
leek, halved lengthwise and sliced
russet potatoes (about 1 1/2 lbs), peeled
medium celeriac (about 1 1/2 lbs), peeled
heavy cream, warmed
ground mace or nutmeg
Cut the potatoes and celeriac into roughly egg-sized pieces, cover with water, add a good pinch of salt, and cook until easily pierced with the tip of a knife, about 10 minutes. Drain and return to the heat for another few minutes, shaking the pan occasionally, to drive off any remaining moisture.
Cook the cabbage and leek in 3 tablespoons of butter with a pinch of salt for about 15 minutes or until tender.
Melt the remaing 5 tablespoons of butter, add it to the potato-celeriac mix, and mash coarsely, then add the cup of cream and mash a bit more. Stir the cabbage-leek mix into the mashed potatoes along with a pinch of mace or nutmeg.
Taste and add salt if needed.