2 onion, very thinly sliced (as thinly sliced as possible, ideally 1mm on a mandolin)
2 tbsp beef dripping
1 tbsp American-style mustard (optional)
salt and freshly ground black pepper
burger seasoning (garlic powder, onion powder, pepper and salt)
3 American or burger cheese slices
3 x 85g/3oz beef mince meatballs (or 4 if you are really hungry)
1 burger bun, split