1 (3- to 3 1/2-pound) brisket
Kosher salt and freshly ground black pepper
4 tablespoons (1/2 stick) unsalted butter
5 pounds (about 10) small yellow onions, peeled and halved through the root
1 cup dry white wine
1 cup veal or chicken stock
1 1/2 tablespoons fresh thyme leaves
1 teaspoon freshly ground black pepper
2 celery stalks, diced
1 tomato, diced
4 garlic cloves, peeled

Preheat the oven to 300F.

Sprinkle the brisket with the seasoning and let stand for 15 minutes. Melt the butter in the bottom of a Dutch oven. Arrange the onions, cut side down, in the pot. Cook over medium heat for about 7 minutes, until well browned on the bottom. Pour the wine and stock over the onions, covering them by 1 inch. Add more stock, if necessary. Add the thyme and pepper. Bring the liquid to a simmer on the stovetop. Place the brisket, fatty side up, on top of the onions. Sprinkle the celery, tomato, and garlic over the brisket. Cover and cook in the oven, turning once, until very tender, about 3 1/2 hours.

Transfer the brisket to a serving platter and boil the braising liquid over high heat until reduced by half, about 15 minutes. Puree the liquid and vegetables in a food processor and then strain through a fine-mesh sieve. Serve the sauce alongside.