4 large onions (about 2 1/2 pounds), very thinly sliced
salt and freshly ground black pepper, to taste
1/4 cup chicken broth
1/2 cup dry red wine
1/2 cup pomegranate seeds
Preheat the oven to 325 F.
In a large heavy-bottomed roasting pan, using two burners if necessary (or in a wide 6-quart Dutch oven or flameproof casserole) over medium-high heat, heat 3 tablespoons oil. Add one 5-pound beef brisket (patted dry), and brown well on both sides, about 10 minutes. Transfer the brisket to a platter and set aside.
Pour off all but about 1 tablespoon of fat remaining in the pan, and add 2 medium chopped onions and 2 chopped leeks. Cook, stirring occasionally, over medium-high heat, until the vegetables are softened, about 5 to 7 minutes.
Add 6 crushed garlic cloves, 2 chopped carrots and 1 chopped celery stalk. Continue cooking until the onions are golden, about 7 to 10 minutes, stirring and scraping the pan to prevent scorching or sticking.
Add 1 cup of pomegranate juice and bring the mixture to a boil, scraping up the browned bits from the bottom of the pan with a wooden spoon, until the liquid is reduced by about half. Add the remaining 1 cup juice, 2 cups chicken broth, 3 thyme sprigs, 2 rosemary sprigs, and 2 bay leaves. Bring the mixture to a simmer. Season to taste with salt and pepper.
Lightly salt and pepper the brisket on both sides and add it to the pan, fat side up, spooning the vegetables all over the meat. Cover the pan tightly (use heavy-duty foil if you don't have a lid for the pan), and braise the brisket in the oven, basting every half hour, until the meat is very tender, about 2 1/2 to 3 1/2 hours. Turn the oven down to 300 F if the braising liquid begins to bubble rapidly.
Cool the brisket in the pan sauce, cover well with foil, and refrigerate until the fat congeals. Scrape off all solid fat. Remove the brisket from the pan and slice thinly across the grain.
Prepare the gravy: Bring the braising mixture to room temperature, then strain it, reserving the vegetables. Skim and discard as much fat as possible from the liquid. Puree the reserved vegetables and 1 cup of the defatted braising liquid in a food processor or a blender. Transfer the pureed mixture and the remaining braising liquid to a skillet and reduce the gravy over high heat to the desired consistency. Taste for seasoning.
Re-warm the brisket in the gravy until heated through. Spread the onion confit over a serving platter and arrange the sliced brisket on top. Ladle the hot gravy over the meat and serve immediately.
About an hour before you are ready to serve the brisket, make the confit. In a 10- to 12-inch heavy skillet, warm 3 tablespoons olive oil. Add 4 large, thinly sliced onions, season lightly with salt and pepper, and toss to coat with the oil. Cook, tightly covered, over the lowest heat, stirring occasionally so the mixture does not burn, for 1 hour, or until the onions are very soft and pale gold in color.
Add additional salt and pepper to taste, 1/4 cup chicken broth and 1/2 cup dry red wine. Raise the heat and boil the mixture, uncovered, stirring, until all the liquid is evaporated and the onions turn golden. Taste and adjust the seasoning (it may take quite a bit of salt), and turn off the heat. Cover the mixture and keep it warm. Stir in 1/2 cup pomegranate seeds just before serving.