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 beef broth to the pot and whisk to remove the browned bits from the bottom of the pot. Add the BBQ sauce and whisk to combine.
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.
Notes
It's best, if you can, to allow the brisket to marinate for at least 30 minutes to lock in the flavor. You can marinate it for up to 24-hours, if you have the time.
Searing the brisket on all sides gives the meat a nice browned and flavorful crust. If at all possible don't skip this step. It adds so much flavor.
Because of the spices in the rub it could begin to burn on the bottom of the Instant Pot.
When deglazing the pot be very careful adding liquid. It will steam up immediately and can burn your arm if you're not careful.