Remove any excess fat from brisket, but leave a little on top. Place one half of the onions on bottom of pan. Season the onions with a litle salt, pepper, garlic powder and lots of Hungarian paprika.
Place brisket on top of onions.
Place other half of onions and allspice on top of the brisket.
Cover tight and roast in a 300 degree oven for 3 hours.
Test your brisket and when it is fork tender, take it out. Let it cool and then slice it very thin. Strain juice and either thicken it with a little cornstarch or serve as is au jus, with onions (as I do).
This is best made in advance. Note: I refrigerate the juices and get rid of the fat that rises to the top. Before serving I pour it over the sliced brisket and reheat in the oven until heated through.