Squeeze 1.5 Tbs juice from lemon into small jar. Add half tsp ginger, 0.75 tsp salt, quarter tsp pepper. Shake until salt dissolves. Add 3 Tbs oil, shake to combine. Remove tough stems from watercress; divide leaves among 4 plates.
In large bowl, beat egg. Drain salmon, add to egg with remaining ginger, scallion, parsley, half cup bread crumbs, half tsp salt and quarter tsp pepper. Mix well and shape into 8 patties. In large non-stick frying pan, heat remaining oil over medium heat. Coat patties with remaining crumbs and cook until golden brown on both sides, 3 to 5 minutes.