Heat the oven to 325°F. Pat the brisket dry and season with salt and pepper (if using a kosher brisket, use a light hand with the salt). In a large Dutch oven or heavy pot, heat 2 tablespoons olive oil; when hot, but not smoking, add the brisket and brown well on both sides. Remove the brisket, lower the heat, and add the chopped onion and leek. Saute until soft. Stir in the flour; saute for a few minutes, then add the wine. Simmer until reduced by half, stirring occasionally. Add the herb packet along with the beef stock. Stir well. Return the meat to the pot; bring to a simmer; cover and place in the oven to cook for 2 ½ to 3 hours, or until fork-tender. During the last hour of the cooking time, add the carrots and celery.
While the meat is cooking, prepare the onions. If using fresh, blanch and peel the onions. In a saute pan, heat the remaining tablespoon olive oil over medium heat; add the onions and saute until golden brown. Season lightly with salt and pepper. Remove from the heat and stir in the balsamic and sugar to form a glaze. Set-aside.
Once the meat is fork-tender and the vegetables are cooked, remove them to a warm serving platter, tent, and keep warm. Remove the herb packet; simmer and reduce the sauce by ½, about 10 minutes. Stir in the caramelized pearl onions, taste, and adjust seasonings.
Slice the meat against the grain; serve with sauce and vegetables. Makes 6 - 8 servings