Heat butter in a Dutch oven. Add leeks, cook gently until just wilted. Add cabbage and combine well. Cook a few minutes. Add potatoes and stock, bring to the boil.
Simmer for 20 minutes or until potatoes are very tender. With potato masher, gently mash some of the potatoes, so that the soup thickens.
Stir in milk, season with salt and pepper. Add green onions and cook 1 minute further.
