Season beef with salt and pepper. Heat the oil in a large skillet and brown the roast on all sides for approximately 5 minutes per side, or until well browned.
Transfer roast to a roasting pan (with a lid) just large enough to hold the beef and the vegetables.
Return skillet to heat and cook the onions, garlic, carrots, cloves and orange zest for about 5 minutes. Use a slotted spoon to transfer the cooked veggies to the roaster, surrounding the meat.
Place the roaster over medium heat on the stove top. Pour in the brandy.
Using a long match or special lighter, carefully set the brandy alight and allow the flames to die down.
Add the beer, peppercorns, bay leaves, thyme and porcini mushrooms. Bring to a boil.
Cover the roaster with foil, then add lid. Transfer to the oven and cook for 2 1/2 to 3 hours, or until roast is done.
Transfer the beef and vegetables to a platter and keep them warm in the oven (you can turn off the heat). Reserve the juices left in the roaster.
For the sauce, mix the vinegar, cornstarch and sugar in a small bowl until smooth. Add 2 TB of the meat juices and mix well. Pour this mixture into the roaster and cook over low heat for 10 minutes, stirring constantly.
Carve the roast and serve with Braised Fennel (Zaar recipe #100458) and boiled new potatoes.