To make the burger quickly fry the chopped red onion and the garlic in some olive oil for 1-2 minutes until translucent.
Add to the rest of the burger ingredients and mix well.
Shape into even size patties and allow too set in the fridge for 1 hour.
Brush a hot griddle or frying pan with olive oil and brown your burgers on both sides for 3-4 minutes until medium, when medium remove from the heat and allow to rest as you would when cooking a steak.
For the onion relish sweat off the onions on a low heat for 10mins then add the sugar and balsamic and continue to cook until all the juice has evaporated.