Take the brisket out of the refrigerator 30 minutes before cooking. Pat dry and lay flat. Cut binding strings and trim down the fat cap, if there is any.
Mix the rub ingredients together in a bowl. Rub evenly on all sides of the meat.
Turn on the saute mode on the Instant Pot. Add the cooking oil. When the pot is hot, add the brisket and brown on both sides, around 4 minutes per side. After 8 minutes turn off the Instant Pot. Remove the brisket to a plate.
Add the BBQ sauce and beef broth. Whisk together, removing the browned bits from the bottom of the pot. Return the brisket to the pot. Scatter the onions on top.
Close the lid and set the vent to sealing. Set to 75 minutes on high pressure. Once the time is up, allow a natural pressure release for 15 minutes.
Remove the meat from the pot. Strain any excess oil from the top of the sauce. Turn on the saute mode and allow the sauce to thicken, whisking occasionally, for about 5 minutes.
If desired, brown the top of the brisket under the broiler for 5 minutes. Slice and serve with the sauce.