Combine the shallots, garlic, salt and pepper in a small nonreactive saucepan over high heat. Stir in the stock and the wine and bring to a boil. Cook over high heat for 15 minutes. Swirl in the butter, remove from the heat, and continue to whisk in the butter until thoroughly incorporated. Mound the potatoes in the center of the plate. Slice a couple of slices off the roast and position them at an angle against the potatoes. Spoon the sauce over top. Garnish with fresh black pepper and long chives.