Put the potato cubes into a large saucepan of salted water, place over a high heat and bring to the boil. Turn down the heat to medium and simmer for 15 minutes or until tender.
Meanwhile, melt half the butter in a sauté pan over a medium heat. Add the onion and fry for 3-4 minutes or until softened but not browned. Add the cabbage and a splash of water, season, cover and cook for 6-8 minutes, until softened.
Drain the cooked potatoes and tip back into the saucepan. Add the rest of the butter, warm milk, double cream, Dion mustard, season well and mash until smooth. Then drain the cooked cabbage of any excess water and stir into the mashed potato, reserving a little of the cabbage mix to garnish the top of the colcannon.
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