Grate potato into a towel and wring to squeeze out excess liquid. Toss potato with salmon and green onions and season with salt and pepper. In a medium saute pan heat oil and add potato mixture, forming it into 8 little round cakes. Cook until golden brown on both sides, about 5 minutes in all. Drain on paper towels and keep warm while you finish sauce.

Meanwhile, make sauce: In a small saucepan melt butter, add leeks and cook until tender. Add potato, cream and water and bring to a boil. Simmer 15 minutes, or until potatoes are tender; adjust seasonings to taste with salt and pepper. Pour into a blender or food processor, add sour cream and process until smooth. Reheat briefly, if necessary. To serve, place 2 cakes on each plate, spoon some sauce around and sprinkle with chopped herbs.