For Veal: Generously season both sides of veal breast with salt and ground black pepper. Place in large, deep roasting pan. Roast in 450°F oven 40 minutes or until well browned.
Remove veal from oven. Reduce oven temperature to 325°F.
Add veal stock, beef broth, wine, coarsely chopped shallots, garlic, thyme, peppercorns and bay leaves to roasting pan. Cover pan tightly with aluminum foil. Braise in 325°F oven 3 to 3-1/2 hours or until veal is fork-tender. Cool slightly.
Remove veal from roasting pan and place on aluminum-foiled-lined sheet pan; reserve braising liquid in pan. While veal is still warm, remove and discard bones and as much connective tissue as possible without tearing meat. Reserve all meat pieces that fall off breast.
Strain braising liquid from roasting pan; cool. Refrigerate, covered, until ready to use. Skim fat from liquid before using.
Cut veal breast in half at center forming 2 equal pieces. Place 1 veal piece, rib side up, on aluminum-foil-lined sheet pan. Layer reserved meat pieces on top. Place second veal piece, rib side down, over meat pieces, lining up 2 pieces evenly (like a sandwich). Cover with aluminum foil, tightly sealing foil around pan edges. Place another sheet pan on top of foil packet and weight down. Refrigerate pressed veal 3 hours or overnight. Cut veal confit into 12 equal pieces.
For Sauce: Sauté mushrooms and finely chopped shallots in 3 oz butter in large sauté pan or rondeau in batches, if necessary, until slightly browned and caramelized. Add braising liquid; bring to a simmer and reduce until medium-thick consistency.
To Serve: Grill 1 piece veal confit until heated through, turning once. Heat 6 oz sauce in small sauté pan; whisk in 1/2 oz butter. Serve sauce with grilled veal.