Preheat the oven to 300°F.

Trim almost all of the excess fat from brisket and lightly sprinkle with salt and pepper. Make a paste of the oregano, thyme, garlic, and olive oil. Spread over both sides of brisket. Let stand at room temperature for 1 hour.

Place brisket in a roasting pan. Spread with onions and then with tomatoes. Cover pan tightly with foil. Bake for 3 to 4 hours, basting every hour with accumulated juices. Remove brisket and tomato mixture to a platter. Degrease pan juices and pour over meat. Refrigerate, overnight for easy slicing. Slice, and reheat with tomato mixture and pan juices and serve.