Preheat oven to 350 and grease a 13x9 baking dish.
Take the pack of rolls out of the bag and cut them in half, width-wise. Place the bottom half in the baking pan. (I like to separate the rolls but it's a personal preference.)
In a small bowl, whisk together melted butter, mustard, horseradish, salt, and sugar. Mix until combined.
Brush the inside of the bottom buns with the buttery mustard mixture, leaving most of the mixture for the topping.
Divide pastrami evenly over each slider bun.
Place about 2 rings of banana peppers on each slider, on top of pastrami.
Divide cheese among all the slices. You can overlap the cheese slices or cut them and place them on each slider bun.
Cover with the top halves of slider buns.
Generously brush each top of the slider bun with buttery mustard mixture.
Bake for 10-15 minutes.
(Leftover rolls can be beautifully reheated in a cooking pan, covered with a lid, over medium heat, for 5-7 minutes. Or, in the oven, covered with aluminum foil.)