准备

• 将土豆放在中锅中；加入足够的冷水以覆盖1英寸。加1茶匙。盐并用中火煮沸。减少热量并慢火煮，直到土豆刚嫩，可以用一把小刀刺穿约25分钟。将土豆转移到盘子上，然后完全冷却（不要冲洗）。
• Cut the green portions from the onion bulbs; reserve the greens. Put the onion bulbs in a medium saucepan with 3 cups of cold water and 1/2 tsp. salt. Bring to a boil over medium heat, and then reduce the heat and simmer until the onions are tender when pierced with a knife, 10 to 15 minutes for spring onions, 4 to 5 minutes for scallions.
• Transfer the onions with tongs to a plate, and then boil the cooking liquid until reduced by half (about 1-1/2 cups), about 2 minutes. Pour the onion broth into a liquid measuring cup and cover to keep warm. Reserve two whole onion tops for garnish, and then finely chop enough of the remaining greens to get a generous 1/2 cup.
• Return the pot to the stove and melt the butter over medium heat. Add the chopped onion greens and cook, stirring, 2 minutes. Stir in the flour and cook, 1 minute. Add the wine, if using, and cook, stirring, until nearly evaporated, about 20 seconds. Gradually whisk in 1-1/2 cups of the reserved warm onion broth, and cook until the mixture begins to thicken, about 5 minutes. Gradually whisk in the cream. Continue whisking until the mixture simmers, about 3 minutes. Reduce the heat slightly and simmer, stirring constantly, until thick and creamy, about 2 minutes. Season with 1 tsp. salt and 1/4 tsp. white pepper, and remove from the heat.
• Position a rack in the center of the oven and heat to 350°F. Peel the cooled potatoes and cut lengthwise into quarters. Add the potatoes and whole onions to the sauce and toss to coat. Transfer the mixture to a 2-quart baking dish, and bake until hot, about 30 minutes. Sprinkle the reserved onion greens on top, and serve.