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 spring onion (reserve one sliced but not fried for garnish) and fry for another 2-3 minutes or until just softened.
Drain the cooked potatoes and then tip back into the saucepan. Add the rest of the butter, warm milk, cream, Dijon, salt and freshly ground black pepper and mash until smooth. Stir the cooked spring onions into the mashed potatoes. Garnish the top with the sliced raw spring onion and serve.
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