In a medium size bowl, combine the zucchini, potato, green onions, flour, egg, egg white, salt, pepper and thyme.
Coat a medium size skillet with cooking spray. Melt the margarine in the skillet over moderate heat. Add the zucchini mixture, shaping it into a cake with a ladle, and cook for about 1 minute.
Coat uncooked side with spray. Turn the cake over and cook for 1 minute.
To serve, cut the cake into quarters. Spoon 1 teaspoon of the horseradish sauce on each quarter, then top with a generous amount of the smoked salmon.