Place the garlic, onion, and corned beef brisket (fat side down) into the Instant Pot.
Place the pickling spice in a spice bag or coffee filter, tie it up tight, and place inside the instant pot. Pour in the beef stock (or beer) and water.
Place the lid on the Instant Pot and make sure that the vent is sealed. Cook on high pressure for 90 minutes, then let stand for 15 minutes before releasing the pressure.
Remove the corned beef from the Instant Pot and cover loosely with foil.
Combine all ingredients for the Thousand Island dressing together in a small bowl and whisk until well combined.
Assemble the sando
Thinly slice the corned beef.
Toast the brioche buns, if desired, then pile high with corned beef and top with a slice of cheese, sauerkraut, and dressing.