Heat pan over medium heat with olive oil. Add all three bell peppers and cook until tender and soft. Set aside.
Slice Hawaiian rolls length-wise with a serrated knife, setting the top of the rolls aside. On the bottom part of the roll, spread Dijon mustard all over. Then place roast beef on top of Dijon. Then add sliced mozzarella and sliced bell peppers.
Place top back on. Melt butter and spread over the top of the rolls. Sprinkle top with everything bagel seasoning.
Bake for 15-20 minutes. Remove from the oven. Slice and enjoy!