Fill a large saucepan about halfway, with warm water. Bring to a simmer. Peel and slice potatoes, and add them to the simmering water. Add a few pinches of salt, increase the heat, and bring to a boil. Check the potatoes with a knife. If the knife has some resistance, continue simmering the potatoes until the knife comfortably slides through the potatoes. When the potatoes have cooked through about 15 minutes, drain them in a large colander. Add them back to the saucepan. To the saucepan, add the milk and butter, followed by a the nutmeg. Mash potatoes until smooth. Stir in Dijon mustard. Season with more salt and pepper, to taste.