Drain the canned salmon, reserving 2 tablespoons of the liquid.
In a mixing bowl, combine the reserved liquid, egg white, breadcrumbs, onion, thyme, pepper and salt. Add the drained salmon and mix well. Shape the mixture into 4 patties about 1cm/1⁄2in thick.
In a frying pan, cook the patties in the oil over a medium heat for 2–3 minutes until brown on the underside. Turn and cook for a further 2 minutes.
Serve each patty warm, with lettuce and tomato, in half a pitta bread.