1 (3 to 3 ½ pound) brisket;
Kosher salt and freshly ground black pepper;
4 tablespoons unsalted butter
5 pounds small yellow onions, peeled and halved through the root
1 cup dry white wine
1 cup veal or chicken stock, or more if needed
1 ½ tablespoons fresh thyme leaves
1 teaspoon freshly ground black pepper
2 celery stalks, diced
1 tomato, diced
4 garlic cloves, peeled
Preheat oven to 300 degrees.
Sprinkle salt and pepper over the brisket and let stand for 15 minutes. Melt butter in the bottom of a Dutch oven. Arrange the onions, cut side down, in the pot. Cook over medium heat until onions are well-browned on the bottom, approximately 7 minutes. Pour wine and stock over onions, covering them by 1 inch. Add more stock, if needed. Add thyme and 1 teaspoon pepper. Bring liquid to a simmer on the stovetop. Place brisket, fatty side up, on top of the onions. Add celery, tomato and garlic to the pot. Cover and cook in the oven, turning once, until very tender, about 3 ½ hours.
Transfer brisket to serving platter. On the stovetop, boil braising liquid over high heat until reduced by half, about 15 minutes. Cool slightly and carefully puree braising liquid in a food processor. Strain through a fine-mesh sieve. Serve the sauce alongside brisket.