Al, trust me on this (but if you don't like lime please skip):
Firstly make sure you buy tinned tuna 'steak' not 'chunks', and try and get it in spring water not brine or oil. Just chop some onion as small as you can get it (maybe less than a quarter of a small onion) and then chop up 2 tomatoes into say 1cm cubes.
Drain the tuna and get it as dry as you can, then simply stir in the raw onion and tomato and a good drizzle of olive oil, some salt and black pepper. Just before eating it (usually on it's own or in a wholewheat pitta) cut a lime in half and squeeze on as much lime juice as you prefer (I use the whole thing).
Takes me about 3-4 mins and it's delicious.