Preheat oven to 350 degrees.
Slice onions thinly into rounds and then cut in half. (Reserve ~1/3 of the sliced onions for the next day)
Mix sauce ingredients in a medium size bowl.
Place brisket in a large baking pan and cover with half of sauce. Refrigerate remaining half of sauce until ready to reheat brisket.
Cook for one hour covered with foil.
Then add 2/3 of the sliced onions* and continue cooking covered for an additional 1 ½ hours. Total covered cooking time 2 ½ hours *(Place remaining 1/3 of onions in a baggie or sealed container and refrigerate until needed the next day when reheating the brisket.)
Let cool and refrigerate overnight covered.
Refrigerating overnight allows the fat to congeal which makes it so much easier to de-fat. Next day, remove fat with a spoon. (See photo)
Before slicing brisket, move onions to the side with a spoon to make the slicing easier.
Then thinly slice the brisket against the grain and put back in pan placing remaining 1/3 of onions on top.
Preheat oven to 350 degrees.
Add remaining sauce, cover and cook for an additional ½ hour. Then uncover and cook an additional 15 minutes to brown the onions.
Serve and Enjoy!