Place the cubed potatoes in a large pot and cover with cold salted water by at least an inch. Bring to a boil over high heat, then reduce to a simmer and cook for 15-20 minutes until the potatoes are tender when pierced with a fork.

While the potatoes are cooking, combine the milk and chopped scallions in a small saucepan. Heat gently over low heat for about 5 minutes to infuse the milk with onion flavor and soften the greens slightly, then remove from heat.

Drain the potatoes thoroughly in a colander and return them to the hot pot for a minute to let excess moisture evaporate. Mash the potatoes using a potato masher or ricer until they are smooth and free of lumps.

Pour the warm milk and scallion mixture into the mashed potatoes, then add the butter and wholegrain mustard. Stir vigorously with a wooden spoon until the mixture is creamy and the ingredients are evenly distributed.

Season with salt and black pepper to taste, adjusting as needed. Serve hot, optionally creating a well in the center for an extra knob of melting butter.