First, fry off the diced onion until soft then add beef mince, black pepper, cayenne and salt. Fry until brown all over and moisture has evaporated. Set aside to cool.
Add a layer of burger bun bottoms to a deep baking tray and spoon half of the mince on top. Add mozzarella and cheddar slices then bacon, gherkins, tomato ketchup and mustard. Repeat this with another layer of the bottom layer of burger buns and then finally top with 6 of the burger bun tops. Brush with egg wash and sprinkle with sesame seeds. Bake for 20 minutes covered with foil then remove foil for final five minutes.