Rinse corned beef under cold water. Place the corned beef in the bottom of an Instant Pot. Pour in 1 cup cold water. Place lid on Instant Pot, make sure steam valve is closed. Set Instant Pot 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, remove from Instant Pot to a sheet pan. With a brush, liberally baste the beef with the glaze.
Preheat oven to 375 - 400 degrees. I use 375 because my oven runs hot. If the oven is too hot, the glaze will smoke leaving it with a bitter flavor. Place the brisket in the oven, basting with more glaze every 5 minutes. Cook for 15 minutes total
Remove brisket from oven and place the brisket on a cutting board. Let rest for 10 minutes, or 1 hour ideally. Switch oven temperature to broil.
With a sharp knife, (not serrated) 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 for just about a minute or until cheese is melted. Top with the other half of the slider bun.
I used Tillamook Swiss Cheese Slices. Fold and divide each slide into 4 equal pieces worked good for the size of the slider buns.