Place the potato chunks in a saucepan of cold salted water. Bring to a boil and cook until soft, about 10-15 mins depending on the size. Drain well, then rice or mash.
While the potatoes are cooking, add the chopped kale and cabbage to a pan with a splash of water. Cover, then steam or sauté for 2-3 mins until just softened. Drain thoroughly, squeezing out any excess moisture, then set aside.
Add the cooked greens to the mashed potatoes along with the Cheddar, Dijon mustard, butter, milk and half of the chopped chives. Stir until smooth and fully combined, then season to taste.
Spoon the mash into a serving dish and sprinkle over the remaining chives.
