Place the peeled and chopped potatoes in a large pot and cover with cold salted water. Bring to a boil over high heat.

Reduce heat to medium and simmer the potatoes for 15-20 minutes, or until they are fork-tender.

While the potatoes are cooking, combine the milk and 2 tablespoons of butter in a small saucepan. Heat gently until the butter melts and the milk is steaming.

Add the chopped scallions to the milk mixture and let them simmer for 2-3 minutes to soften and infuse the liquid. Remove from heat.

Drain the cooked potatoes well and return them to the hot pot to dry out slightly for a minute.

Mash the potatoes thoroughly using a potato masher or ricer until smooth.

Pour the hot milk and scallion mixture into the mashed potatoes and stir to combine.

Fold in the wholegrain mustard and grated cheddar cheese until the cheese is melted and the mixture is creamy.

Season with salt and plenty of black pepper to taste.

Transfer to a serving bowl and create a well in the center. Place the remaining 2 tablespoons of butter in the well to melt before serving.