In a large pot cover the potatoes with water and bring to a boil. Reduce and simmer until they are fork tender. Drain.
Meanwhile, while potatoes are cooking; melt 2 tbls butter and the olive oil in a large saucepan.
Add onion, garlic and leeks and cook, stirring frequently, until very soft and just beginning to brown around the edges, about 10 minutes.
Add the cabbage and cook, stirring constantly, until completely wilted and soft. Season with salt and pepper.
Combine the cabbage mixture, boiled potatoes, milk, cream, remaining butter, scallions, parsley and coarsely mash with a potato masher. Season with salt and pepper.
