Finely slice the red onion; this is best achieved on a mandolin or in the food processor, then pop into a bowl with a good drop of the red wine vinegar and a pinch of salt. Scrunch the onion up to make a quick pickle and leave to one side.
If you’re making 4 adult sized burgers, mix the meat up then mould into 4 burgers, you may want to make 2/3 large burgers and use the remaining mince to form smaller child friendly burgers. Chop the white onion and apple finely and mix into the mince, Season with salt and pepper and mould away, then fry on high heat for 4 minutes, then flip over and crumble over your cheese and cook for another 4 minutes or until thoroughly cooked.
We use homemade Brioche Buns ….Cut your buns in half, toast them if you like. Top your cheesy burger with some spinach and then squeeze the excess vinegar out of your soaking onions and top your burger with this. Bun lid on and you’re done… But I would always add ketchup!