PREP: Generously grease a 9x13-inch pan with cooking spray. Preheat oven to 350 degrees F.
ONION and PEPPER: Add oil to a large skillet over medium-high heat. Once hot, add sliced pepper and onion. Sauté, stirring occasionally until tender and slightly charred all over, about 8-10 minutes. Remove and set aside to cool.
ASSEMBLE: Slice the slab of rolls in half horizontally, keeping individual rolls intact. Spread the mayo evenly across the inside surfaces of both the base and the top of the rolls. Place the base bread in the pan and top with 6 slices of cheese. Add roast beef on top then peppers and onions. Add the remaining 6 slices of cheese on top of the veggies.
BAKE: Bake WITHOUT the top for 10 minutes, to let the cheese melt. Meanwhile, mix together all “butter” ingredients. Whisk until smooth.
ADD BUTTER: Remove pan from oven and add the top slab of rolls. Pour the butter all over, brushing generously on top and on the sides of the rolls. Be sure to cover all the bread (except the bottom).
BAKE AGAIN: Cover with foil and bake for 20 minutes. Remove foil and bake another 5-10 minutes or until golden brown on top and cheese is melty.
ENJOY: Cut and serve while hot. Enjoy!
Video
Notes
Note 1:Beef: Request the rarest roast beef at the deli counter. This ensures it remains tender and not overcooked after baking. For less meaty sliders, use ¾ lb of roast beef, or 1 lb for meatier ones.