Preheat oven to 325 degrees.
In a heavy pot or Dutch oven heat olive oil and sear the brisket until lightly browned on all sides. Remove meat from pan and let cool for several minutes. In a small bowl, combine rosemary, 2 tablespoons thyme and garlic. Rub the seared brisket all over with the herb mixture. Place leeks in the bottom of the Dutch oven and place brisket on top. Pour wine and tomatoes around brisket and sprinkle with peppercorns. Bake, covered, for 3 hours, basting frequently. Make sure the meat is fork tender when you remove it from the oven. Cool, place in refrigerator overnight and remove congealed fat from surface.
To make sauce: Remove meat. Reduce the liquid in the Dutch oven by half. Strain the liquid into a saucepan and heat to a simmer. Whisk in fresh thyme leaves and salt and pepper, to taste. Return brisket to sauce and reheat before serving. Serve over egg noodles or potato latkes.