1. Preheat the oven to 325℉. Rub Old Bay Seasoning into both sides of the brisket. Set the brisket fat side up on a large piece of foil and wrap tightly. Transfer to a shallow roasting pan and bake until tender, about 4 hours.
2. While the brisket is cooking, combine mayonnaise with caramelized onions and black pepper. Allow the flavours to marry for 30 minutes before using.
3. After the brisket has cooled slice it thinly. Spread each slider bun with 1tbsp of the seasoned mayonnaise, top with 4oz sliced brisket and half a slice of cheddar cheese.