Place potato in a large saucepan. Add stock and water as needed to cover. Boil potato until tender, 20 to 25 minutes.
Place leek, milk, garlic and bay leaf in a large saucepan. Cover. Bring to the boil and simmer until leek is softened, 20 minutes. Drain; reserve leek and the milk and garlic liquid separately. Discard bay leaf.
In the same saucepan, place cabbage and ¼ cup (60 ml) water. Cover. Boil gently until tender, 10 to 15 minutes. Drain. Squeeze cabbage dry; chop finely.
Drain potato; transfer to a large bowl. Add milk and garlic and mash well. Stir in leek, cabbage, nutmeg, salt, pepper and butter. Top with chives.