Combine onions, parsley, garlic, bay leaves, thyme, cloves and olive oil in non-reactive container. Place veal brisket in marinade; turning to coat. Cover and marinate in refrigerator 6 hours or as long as overnight; turning occasionally.
Remove brisket from marinade and discard marinade.
Season brisket with salt and pepper; brown on both sides in roasting pan. Set aside.
In same roasting pan sauté mushrooms, carrots, bacon and celery until lightly browned. Add tomatoes and sauté until tomatoes are softened.
Deglaze pan with white wine and simmer for 2 minutes; stir in barbecue sauce. Return brisket to pan; cover tightly with foil and braise in 350ºF oven for 2-1/2 to 3 hours or until brisket is fork-tender. Remove brisket from pan; reserving braising liquid and vegetables. Carve brisket into thin slices; set aside and keep warm.
Place 6 oz. brisket slices in center or plate. Ladle about 4 oz. braising liquid with vegetables around the slices. Garnish with thyme sprig.