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Preheat the oven to 300ºF. Have ready a baking dish or roasting pan large enough to hold the brisket.
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Rub the brisket with sumac, mint, and a sprinkling of salt and pepper.
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Warm the oil in a large skillet over medium-high heat. Sear the brisket, fat side down first, until browned, about 5 minutes per side. Transfer the brisket to the baking dish.
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Pour off all but a couple of tablespoons of fat from the skillet. Lower the heat to medium-low and add the onions, garlic, and a sprinkling of salt. Cook, scraping up any brown bits back into the onions, until the onions are softened, 7 to 10 minutes.
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Stir in the pomegranate molasses, tomatoes, 1 cup of the stock, and 1/2 teaspoon salt. Raise the heat and bring to a boil. Lower the heat to a rapid simmer and cook for 5 minutes to marry the flavors.
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Pour the liquidy onions over the brisket. Distribute the plums around the pan along with the bay leaf and thyme. Cover tightly with foil and transfer to the oven.
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Roast, flipping the brisket every half hour or so, until the meat is completely tender, 3 1/2 to 5 hours, depending on the size. (Test by inserting a fork into the brisket.) If you are using a large roasting pan, add the second cup of stock or water after an hour or so. Remove the bay leaf and thyme sprigs. Cool, skim any fat off the top, and slice the meat against the grain. Reheat in the gravy to serve.