Form two patties out of the beef mince and season with salt and pepper.
Place a cast iron pan or frying pan, over a high heat and allow it to preheat for a minute.
Press each patty to a thickness of about 2cm. Fry for 4 minutes on each side, adding the american cheese for the final two minutes of fry time.
Remove both patties from the pan, then add a thin layer of mayo on to each slice of bread.
Place two pieces of bread, mayo-side down, back into the pan over a medium/low heat, then add a layer of mozzarella, the cheesy burger patty, a drizzle of mustard, and the other piece of bread, with the mayo facing outwards.
Fry each side for 2 minutes, or until nicely grilled and melted. Serve immediately & enjoy! Optionally top with fresh parsley.