Boil the potatoes in a pot until tender. While this occurs, preheat the oven to 425 degrees F. Place the sablefish in a parchment-lined baking dish. Drizzle with 1/4 cup of the wine and 1 Tbsp of the butter; season with salt and pepper. When the potatoes are ready to be mashed, cover the fish with foil and bake 8 to 10 minutes, or until just cooked through. Mash the potatoes, and then whip in the butter, buttermilk, green onions, salt and pepper. Cover and keep warm over low-heat until the fish and sauce is ready. To make sauce, place the remaining wine in a small pot. Bring to boil and reduce by half. Add the cream and reduce until the sauce lightly thickens. Stir in the mustard and salt and pepper to taste. To serve, mound a scoop of the potatoes in the centre of a dinner plate, set on the fish, drizzle with some of the sauce and serve.