Season the mince with about ½ teaspoon salt and a few grinds of black pepper. Tumble and mix loosely (don't overwork this mix). Split the beef and form into two 100g balls and refrigerate for a few minutes while you prepare everything else.
Get a shallow pan over a medium-low heat. Peel and slice the onion into rings. Add a splash of oil and fry the onion gently for about 10 minutes until softened and starting to gently colour. You're just breaking the onion down a bit.
Wipe out your pan with kitchen roll and put the heat up to high. Take your meatballs and put them in the pan, pressing down hard with a flat tool like a fish slice to squidge them into patties just under a centimetre thick. Leave undisturbed to colour for 3-4 minutes. When browned flip to complete cooking for another minute or two. The patty should be thin enough it won't take much cooking by the time it's coloured.
While the meat cooks, split and toast your bun as desired. When everything is ready it's time to assemble. From the bottom up: bun, beef, cheese slice, beef, cheese slice, grilled onions, crispy onions, cheese sauce. Eat immediately.
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Notes
This recipe is a great knock off of the McDonald's limited edition burger you can make at home. You can make the patties in advance and freeze.