Place cubed potatoes in a large pot. Fill with water to just cover the potatoes. Add salt. Turn heat on, and allow to simmer until tender, about 10 minutes. Drain the water from the potatoes, and mash.
Heat a sauté pan on medium high heat. Add Oil. Sweat the onions and ginger until the onions become translucent, about 7 minutes.
Add milk or cream. Do not allow the cream to boil, but make sure it is hot. Add the Wasabi Mustard and mix thoroughly. Add the butter, and stir until melted.
Pour mixture into mashed potatoes and mix thoroughly. Add salt and pepper to taste. Garnish with scallions.