Put the beef, chives, egg, onion, breadcrumbs and a pinch of salt and pepper in a mixing bowl. Mix well with your hands then divide into four and shape into burgers. Put on a plate and put in the fridge to chill for about half an hour.
Put the sliced cucumber in a glass bowl. Heat the sugar, water, salt and vinegar in a pan until dissolved then leave to cool before pouring over the cucumber. Leave to pickle for about 20 minutes.
Season and char the outside of each mushroom on a skillet or barbecue then set aside. Cook the burgers to your liking on the chargrill.
Drain the cucumbers from the pickling liquid.
Put a burger on top of a mushroom ‘bun’, add a little pickled cucumber and a handful of rocket. Put the top of the ‘bun’ on and serve with ketchup and mustard.
