Slice the onion into thin rings and place in a bowl. Bring the white wine vinegar and sugar to the boil in a saucepan and stir until the sugar dissolves. Pour over the onion rings and leave to cool.

Finely chop the salmon with the shallot and capers in the food processor or chopper. Mix with the flour and season with salt and pepper. Form the salmon mixture into patties.

Halve the avocados, remove the pit and peel off the skin. Using a stick mixer, puree half of the avocado(s) with the aioli until smooth. Slice the remaining avocado(s). 

Heat the oil in a frying pan. Fry the salmon burgers for 3 minutes on each side until brown.

Halve the hamburger buns. In the same pan, toast the buns with the cut side down for 2 minutes or until golden brown and crispy.

Spread the avocado aioli on the bottom half of each bun. Divide some salad leaves and/or watercress over the aioli. Top with the salmon burger patties, avocado slices and pickled onions. Garnish with some extra salad leaves and/or watercress if desired and cover with the top halves of the buns.

Special Equipment
·       Food processor or chopper, stick mixer