Drain the salmon and break into large flakes in a bowl. Add the mashed potatoes, spring onions, lemon juice and zest and chopped parsley. Season, then mix together to bind.
Form into thick fishcakes on a lightly floured board. Chill for 30 minutes in the fridge.
Heat the oil in a pan and fry fishcakes for five minutes each side, or until browned. Crisp and heated through.
Garnish with lemon slices and served with a tossed salad.