Remove the skins from the potatoes, grate the potatoes coarse into a large bowl, and add the onion mixture. In a small bowl whisk together well the mayo, the egg, the Worcestershire sauce, the thyme, the capers, and S & P to taste. Add the mixture to the potato mixture, and stir the mixture until it is combined well. Add the salmon and stir the mixture until the salmon is just distributed evenly. Form 1/3-cup measures of the mixture into patties and dredge the patties in the flour. In a large skillet heat the butter and the oil over moderately high heat until the fat is hot but not smoking and in the fat saute the patties in batches for three minutes on each side, or until they are golden brown. Line a platter with the lettuce and serve the salmon cakes with the lemon. Serves 6