Mix the mustard, brown sugar, and packet of spices together and coat the brisket with the mixture. Can be done ahead of time, marinate for up to 8 hours. Place the cut onions in the bottom of the Instant Pot, place the corned beef over the onions, fatty side up. Put the entire box of broth in, lock on lid, set toggle to sealing. Cook on manual/pressure cook for 90 minutes. Quick release the pressure, remove beef and onions to plate and cover with foil.