Mix together the tuna, onion, egg and bread crumbs and shape into two patties, fry in butter. In the meantime, place two thick slices of tomato on a plate. While your patties cook, mix together the mayo, milk, chopped cucumber,
sea salt, pepper and what other seasonings you may like. When the tuna patties are done, put them on the tomatoes and spoon cucumber mixture over the top.