Place cabbage and 2 cups water in a large saucepan; bring to a boil. Reduce heat; simmer, covered, until cabbage is tender, about 10 minutes. Drain, reserving cooking liquid; keep cabbage warm in separate dish.

In same pan, combine potatoes and reserved cooking liquid. Add additional water to cover potatoes; bring to a boil. Reduce heat; cook, uncovered, until potatoes are tender, 15-20 minutes. Meanwhile, place milk, green onions, salt and pepper in a small saucepan; bring just to a boil and remove from heat.

Drain potatoes; place in a large bowl and mash. Add milk mixture; beat just until blended. Stir in cabbage. To serve, drizzle with butter; top with parsley and bacon.