Rinse corned beef under cold water. Place the corned beef, fat side up, in the bottom of an Instant Pot. Sprinkle on the seasoning packet and pour in 1 cup cold water. Place lid on Instant Pot and set to cook on high pressure for 80 minutes.
For the glaze. Mix together the mustard, horseradish, vinegar and molasses. Set aside.
When corned beef is done, unplug the Instant Pot and let the steam naturally release for 10 minutes. Then press the steam valve to release the remaining steam. Remove corned beef to a sheet pan. Using a silicone brush, liberally baste the corned beef with the glaze.
Preheat oven to 400 degrees. Place the brisket in the oven, cook for 15 minutes basting with glaze every 5 minutes. Lower oven heat if you feel it's too hot for the glaze. If it smells like it's burning, turn it down.
Remove brisket from oven and place the brisket on a cutting board. Let rest for 10 minutes. Switch oven temperature to broil.
With a sharp knife, slice the meat against the grain. Place chunks of corned beef onto the bottom of slider buns. Top with slices of Swiss cheese. Add a small drizzle of additional glaze if you'd like.
Broil briefly until cheese is melted. Remove from oven, top with the other half of the slider bun and serve.
Notes
Tips For Success
Swiss cheese is not the most cooperative melting cheese ... unless you let it come to room temperature. Set the cheese out for about 30 minutes to soften.
Rinse: Rinse the corned beef with cold water before cooking. The reason is to remove the salt and brine. Rinsing helps insure the meat is not too salty, but doesn't hurt the flavor, which is already infused into the meat.
Don't Rush It: Brisket doesn’t care about your timeline, please make sure the meat is nice and tender.
Broiler: Watch those sliders like a hawk once they're in the broil stage. The molasses glaze is sugary and will burn quickly.