Preheat oven to 350F.
Season veal generously with dried thyme, salt and pepper.
Heat oil in heavy large skillet over high heat.
Add onions and sauté until brown, about 15 minutes.
Sprinkle flour over and stir 3 minutes.
Transfer mixture to large roasting pan, spreading onions in center and bones around edge.
Add 1 brisket to same skillet and cook until brown, about 5 minutes per side.
Transfer brisket to roasting pan and place atop onions.
Repeat with second brisket.
Add button mushrooms, stock, Marsala, dried mushrooms, garlic and allspice to same skillet.
Bring mixture to boil, scraping up browned bits.
Pour contents of skillet over briskets.
Cover tightly with heavy-duty foil.
Roast until briskets are very tender, about 2 hours.
Uncover; let cool at least 30 mins.
Transfer briskets to platter.
Spoon onions into strainer set over large saucepan.
Press onions hard to release juices.
Discard onions in strainer. Set mushrooms aside.
Strain pan juices in roasting pan into same large saucepan.
Spoon fat from top of pan juices and discard.
Boil until pan juices coat spoon lightly, about 20 minutes.
Return reserved mushrooms to sauce.
Season to taste with salt and pepper.
Cut briskets diagonally across grain into thin slices; trim fat, if desired.
Overlap slices in large casserole.
Spoon mushroom sauce over.
(Can be prepared 3 days ahead.)
Cover with foil and refrigerate. Rewarm covered in 350F oven before continuing, about 45 minutes.
Garnish with fresh thyme, if desired, and serve.
