How to Make It
Bring a large saucepan of water to a boil. Add the onions and blanch for 30 seconds. Drain in a colander under cold running water. Slip off the papery skins and pat the onions dry.
In a large, deep skillet, combine the chicken stock with the water, butter, vinegar and sugar. Cover and bring to a boil. Add the onions, cover and cook over moderately low heat until crisp-tender, about 15 minutes. Uncover and cook over moderate heat, stirring occasionally, until the onions are covered with a deep-golden caramel, about 30 minutes longer; add a few tablespoons of water as the liquid evaporates.
Add the Cognac, prunes and chestnuts to the onions and cook just until heated through. Season with salt and pepper and transfer to a serving bowl. Add 2 tablespoons of water to the skillet and scrape up any caramel stuck to the bottom and sides. Pour the sauce over the onions, garnish with the parsley and serve.
Make Ahead
Notes
Don t waste time peeling cipollini individually, the way you would large onions. First, blanch them in boiling water for about 10 seconds. Drain, rinse in cold water and drain again. Trim off the roots and the skins should slip off easily. You can also simply soak the onions in warm water for about half an hour to get the same loosening effect.
For chestnuts in vacuum-sealed jars, we recommend Verfeuille Whole Chestnuts, from the south of France, which are roasted and lightly steamed ($9 for 8 oz; Fairway Market, 212-595-1888) or Trader Joe's Whole Chestnuts, from Spain, which have an intense flavor and luxurious texture that belies their bargain price ($4 for 14 oz).