Place sweetbreads in large saucepan. Add wine and just enough water to cover. Add bouquet garni to pan. Simmer 30 minutes. Remove sweetbreads and trim off any odd bits. Cut into bite-sized pieces and reserve.
Sauté onions, garlic, Swiss chard and mushrooms in 1/4 cup olive oil; 5 minutes. Add sherry and 1 cup veal stock. Simmer 20 minutes. Stir in tarragon and reserved sweetbread pieces. Season with salt and pepper.
Spread stuffing mixture inside shoulder pocket. Roll and tie shoulder. Season with pepper. Sauté veal in 3 tablespoons olive oil until browned on all sides, about 5 minutes. Deglaze pan with veal stock. Stir in reserved tarragon stems. Cover pan and poach 1-1/2 to 2 hours or veal is fork tender. Remove veal; keep warm. Strain pan juices.
Heat 1/2 cup pan juices in small skillet. Swirl in 1 tablespoon butter; reduce slightly. Plate one 4 to 5 ounce slice veal shoulder on plate. Spoon sauce over veal.