Place cubed potatoes in a large pot. Fill with water to just cover the potatoes. 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. Heat one minute more. Sweat the onions and ginger until the onions become translucent, about seven minutes.
Add cream. Do not allow the cream to boil, but make sure it is hot. Add the Stonewall Kitchen Wasabi Mustard and mix thoroughly. Add the butter, and stir until melted.
Pour cream mixture into mashed potatoes and mix thoroughly. Add salt and pepper to taste. Garnish with scallions.