Combine spices in a small bowl. Rub over the entire surface of the brisket. Set aside.
Heat the oil in the Instant Pot on Sauté mode.
Once hot, add the onions and garlic. Cook for 2-3 minutes until slightly softened, stirring frequently to not allow garlic to burn.
Turn off Sauté mode.
Place the brisket on top of the onion mixture in the Instant Pot. You may need to cut it in half to fit properly.
Pour the beef broth and BBQ sauce into the pot.
Close the lid on the Instant Pot, turn the valve to the "sealing" position.
Set the Instant Pot for 90 minutes on "Meat/Stew" mode (or pressure cook high if your Instant Pot doesn't have that setting).
Once done cooking, let the Instant Pot naturally release for 10 minutes then turn the valve to "venting" and quick release the rest of the pressure.
Remove the lid and use tongs to remove the brisket from the pot to a cutting board.
Slice or shred the brisket depending on preference.
Turn the Instant Pot to Sauté mode and cook, reducing the sauce left in the pot for 5 minutes or so until slightly thickened. Whisk consistently as it cooks.
Add the brisket back to the pot with the sauce to coat or, plate the brisket and spoon the sauce on top before serving.