To make the burgers, place all the ingredients in a bowl and mix thoroughly with your hands. Shape into 4 round, even patties and place in the fridge to firm up for 10 minutes.
Finely slice the onions and cucumber and place into individual bowls. Divide the vinegar between the bowls, making sure the vegetables are fully submerged in the liquid. Leave to pickle for 15 minutes, then drain.
To cook the burgers, heat up a barbecue or griddle pan. Lightly oil the burgers and cook for about 4–5 minutes on each side, or until nicely browned on the outside and cooked inside.
Slice the buns and lightly toast on the barbecue or griddle pan.
Spread your condiment of choice on the buns then top with a burger, a slice of tomato and plenty of pickled onion and cucumber. Close the burgers and serve with the remaining pickles on the side.