8 stalks of green garlic, white and light green parts only, finely chopped
1/4 cup heavy cream
1/2 teaspoon white wine vinegar
Preheat the oven to 400°. In a small baking dish, drizzle the potatoes with olive oil. Season with salt and pepper and roast the potatoes for about 30 minutes, until tender.
In a small saucepan, melt the butter. Add the water and green garlic, cover and cook over low heat until the garlic is tender, about 4 minutes. Add the cream and simmer over moderate heat until lightly thickened, about 3 minutes. Stir in the vinegar and season the sauce with salt and pepper.
Transfer the potatoes to a bowl. Pour the sauce over the potatoes and serve.
The sauce can be refrigerated overnight. Reheat gently.