Veal brisket is the boned veal breast. If you don't have a roasting pan large enough to hold both briskets, divide the ingredients in half and bake in two pans. If the veal is difficult to find, substitute one 5-pound flat-cut beef brisket and roast until tender, about 3 1/2 hours. Leftovers freeze well and make great sandwiches.
Preheat the oven to 350 degrees. Season the veal generously with the dried thyme, salt and pepper. Heat a heavy, large skillet over high heat. Add the veal bones and cook until some fat is rendered, turning occasionally, about 5 minutes. Add the onions and cook, stirring, until brown, about 15 minutes. Add the flour and stir 3 minutes. Transfer the mixture to large roasting pan, spreading the onions in the center and the bones around edge. Add 1 brisket to the same skillet and cook until brown, about 5 minutes per side. Transfer the brisket to the roasting pan and place on top of the onions. Repeat with the second brisket.
Add the mushrooms, stock, Marsala, dried mushrooms, garlic and allspice to the skillet. Bring the mixture to a boil, scraping up any browned bits in the pan. Cover tightly with heavy-duty foil. Roast until the briskets are very tender, about 2 hours. Uncover and cool at least 30 minutes.
Transfer the briskets to a platter. Remove the bones from the pan and discard. Spoon the onions into a strainer set over a large saucepan. Press the onions firmly to release the juices. Discard the onions. Set the mushrooms aside. Strain the pan juices in the roasting pan into the saucepan. Skim the fat that has risen to the surface of the pan juices and discard. Boil the pan juices until the mixture has thickened enough to lightly coat the back of a spoon, about 20 minutes. Return the mushrooms to the sauce. Season to taste with salt and pepper.
Cut the briskets diagonally across the grain into thin slices; trim off the fat, if desired. Overlap the slices in a large casserole. Spoon the mushroom sauce over the meat. (Can be prepared 3 days ahead. Cover with foil and refrigerate. Re-warm covered in 350-degree oven before continuing, about 45 minutes.) Garnish with fresh thyme, if desired, and serve.
Makes 8 servings