Make a day ahead, or start it on the morning of the day you plan to serve it, to allow plenty of time for chilling.
Trim as much fat as possible from brisket. Combine meat with bones and water in Dutch oven. Heat until gently simmering. Add onions, leeks, carrots, parsley, thyme, and bay leaf. Cover and simmer 2 hours. Discard vegetables. Remove meat and cut into serving pieces. Discard bones. Pour stock into large bowl, cool to room temperature, refrigerate, return meat and stock to Dutch oven and reheat. As stock is heating, steam or boil carrots and potatoes in separate pans until tender. Add to stew. Season with salt and pepper.