Bring a pot of salted water to a boil and boil the cabbage until tender, about 12-15 minutes. Drain off the water and chop the cabbage. Set aside.
Bring another pot of water to a boil and boil the potatoes until tender. Drain off the water and set aside. Put the leeks in a saucepan.
Cover with the milk, bring close to boiling and then turn down to a simmer until tender. Set aside. Add the mace, salt and pepper and garlic to the pot with the potatoes and mash well with a hand masher.
Now add the leeks and their milk and mix in with the potatoes, taking care not to break down the leeks too much. Add a little more milk if necessary to make it smooth.
Now mash in the cabbage and lastly the butter. The texture that you want to achieve is smooth buttery potato with interesting pieces of leek and cabbage well distributed in it.
Transfer the whole mixture to an ovenproof dish, make a pattern on the surface and place under the broiler to brown.
