1. Boil potatoes in salted water until tender, drain well and return to pot.
2. Finely slice onion and simmer gently in milk until the onion is soft, drain, reserving the milk.
3. Meanwhile remove stalks from kale and roughly chop. Place kale in a pot with one inch of boiling, salted water. Cover and cook for 3-5 minutes until kale is tender but still bright green, remove from heat and drain.
4. Mash potatoes with three quarters of the butter, add the onion and kale and beat together with a wooden spoon. Gradually add the milk until the potatoes are soft and creamy. Serve immediately with remaining butter on top.