Heat the oil in a small frying pan and fry the onion for five minutes until softened. Leave until completely cold. Place the mince in a mixing bowl and mix in the cooled onion, garlic, American seasoning, tomato ketchup and parsley.

With damp hands shape the mince into 4 even-sized burgers. Heat the oil in a frying pan and fry the burgers for 6-7 minutes each side. Alternatively grill the burgers on a hot barbecue for 6-7 minutes per side. Split the burger buns and place a pile of watercress on the base of each bun. Lay a burger over and top with a slice of cheese. Arrange a few slices of tomato over the cheese and top with a bit more watercress. Place over the top of the bun and serve immediately with extra ketchup and a few mini gherkins (cornichons) on the side.