Place the peeled potato chunks in a large pot and cover with cold salted water by about an inch.

Bring the water to a boil over high heat, then reduce to a simmer and cook for 15-20 minutes until the potatoes are very tender when pierced with a fork.

While the potatoes cook, combine the milk and chopped scallions in a small saucepan over medium-low heat.

Simmer the milk mixture gently for about 5 minutes to soften the scallions and infuse the milk with their flavor, then remove from heat.

Drain the potatoes thoroughly in a colander and return them to the hot pot to dry out for a minute.

Mash the potatoes until smooth using a potato masher or ricer to ensure there are no lumps.

Pour the warm milk and scallion mixture into the mashed potatoes, adding the butter, whole grain mustard, salt, and pepper.

Stir everything together vigorously with a wooden spoon until the mixture is creamy, fluffy, and well combined.

Taste and adjust seasoning if necessary, then transfer to a serving bowl.

Serve immediately, optionally creating a well in the center with an extra knob of melting butter for a traditional presentation.