Preheat the oven to 350 degrees F.
With a knife, pierce the skin of the brisket in 5 places and insert the garlic cloves. Sprinkle with salt and pepper. Heat 2 tablespoons of the oil in a heavy skillet or roasting pan, add the meat, sear on all sides and remove.
Add 2 more tablespoons of the oil to the same pan and saute 3/4 of the onions (about 6 cups) until they are limp. Add the turmeric, ginger, white pepper, bay leaves, celery, 1/3 of the diced tomatoes, and water to the pan. Stir-fry a minute or 2 and let cool.
Place the brisket in a baking pan and surround with the cooked vegetables. Roast, covered, in the oven for 3 hours or until a fork goes in and out of the meat easily. Remove, cool and refrigerate, reserving the vegetables. You can prepare this a day ahead of time.
Tomato-Onion Sauce: Heat the remaining tablespoon of oil in the frying pan; add the remaining onions and saute until onions are translucent. Then add the remaining diced tomatoes and simmer, covered, for a few minutes. Set aside or refrigerate overnight or until ready to serve the meat.
When ready to serve, remove any fat that accumulated on the brisket as it cooled. Cut, against the grain, into slices about 1/4-inch thick. Return the slices to the baking pan along with the reserved vegetables in which the meat was cooked. Preheat the oven to 350 degrees F and reheat the brisket, covered, for about 30 minutes.
Add to the tomato-onion mixture, olives, preserved lemons and 2 tablespoons each of parsley and cilantro and heat in small saucepan. Remove the brisket and some, or all, of the vegetables to a serving platter and serve, covered with the tomato-onion sauce and garnished with the remaining parsley and cilantro.
Cut 7 of the lemons lengthwise, almost into quarters, leaving them intact at one end. You can also slice them thin.
Using your fingers, stuff as much salt as possible inside the lemons, close them, and place in sterilized wide-mouth 2 quart jar. Squeeze the juice of at least 4 lemons into the jar. Allow to stand, half covered, at least 1 week on the counter, shaking the bottle each day, or until the peels sink with the weight of the salt in the jar. Then add a few more salted lemons, lemon juice, and, if you like, olive oil to cover.
Close the jar and leave out on the counter for at least 3 weeks before using. When using the lemons, merely rinse with water, remove the seeds, and chop up for your recipes. Refrigerate after opening.