Put salt and pepper on the brisket pieces and let it sit while you get your other ingredients together.

In a small cup or bowl, mix together all the ingredients for the sauce.

Place sliced onions in the inner liner of your pressure cooker or Instant Pot. Place the sliced beef on top.

Pour over the mixed sauce.

Cook the meat at high pressure for 60 minutes for a brisket with some chew, and at 70–75 minutes for a more tender brisket.

Open the pot and remove the brisket with tongs.

Using an immersion blender, blend together the onions and all the liquid in the pot. Add mustard and ensure it is mixed well.

Turn your Instant Pot to sauté and add the xanthum gum or corn starch and allow the sauce to thicken.

Slice the brisket, and serve with the sauce.