Preheat your oven to 375°F (190°C). Lightly butter a medium floral or pretty ceramic casserole dish so the potatoes don’t stick and the edges brown nicely.
Place the peeled, chunked potatoes in a large pot and cover them with cold water by about an inch. Add a generous pinch of salt if you like. Bring to a boil over medium-high heat, then reduce to a gentle boil and cook until the potatoes are very tender when pierced with a fork, about 15–20 minutes.
While the potatoes cook, prepare the cabbage. Finely shred the green cabbage with a knife, aiming for thin ribbons so it turns tender in the mash and bakes up soft and sweet.
In a small saucepan or microwave-safe measuring cup, gently warm the whole milk with 4 tablespoons of the butter until the butter is mostly melted and the milk is hot but not boiling. This helps keep the potatoes fluffy instead of gluey.
Drain the cooked potatoes very well and return them to the warm pot. Let them sit for a minute or two so excess steam can escape; this keeps the mash from getting watery.
