Season the beef mince with the pepper and the garlic powder. Mix well then transfer to a lined board and shape into a rectangular patty the same size as your loaf. Chill for 15 minutes in the fridge.
Fry your patty over high heat in the olive oil. About 1-2 minutes on each side, depending on how you like your burger done! Set aside.
Cut the very top and bottom off the loaf - exposing the bread 'flesh'. Cut the loaf in half lengthwise. Scoop out half of the flesh from each loaf half (save for breadcrumbs or just nosh it).
Spoon caramelised onions and bbq sauce into each crevice. Divide the red leicester and cheddar between each half. Top the bottom half with the patty, then top with the other.
Melt 2 tbsp butter in a large frying pan over medium heat. Place the loaf in. Top with a piece of greaseproof paper and weigh down with something heavy. Fry for a good 5 minutes - or until the underside is a satisfying, crisp golden brown. Carefully flip the sandwich and repeat the weighing process. Cook until you see cheese arriving at the edges of the sandwich. If necessary, pop in a hot oven for 5 minutes until the inner sandwich temperature reads around 60°C (for the cheese - you have already cooked your burger!).