Place chuck roast in your slow cooker, sprinkle on garlic powder, salt & pepper, pour in jar of pepperoncini. Set to low and cook for 8-10 hours.
Shred beef with forks and slice pepperoncini.
Preheat oven to broil.
Stir together mayonnaise and cream cheese.
To serve, pile beef & peppers on bottom half of slider buns, top with pepper jack cheese. Spread a spoonful of mayonnaise mixture on top half of bun. Quick trip under the broiler until cheese has melted.
Replace top of bun, ladle a spoonful of the cooking liquid into a side dish for dipping.