Pre heat the oven to 200C gas 6.
Heat the olive oil in a medium sized deep non stick frying pan, add half the potato slices overlapping slightly.
Cook for a couple of minutes to take a little colour.
Sprinkle on the raw onion and tuna fish chunks evenly.
Sprinkle over the warmed, cooked rice.
Top with the rest of the cooked potato.
In a bowl, whisk together the eggs, milk, olive oil, oregano and then season well.
Pour over the potato mixture, and place in the hot oven until risen and set, about 10-12 minutes.
When cooked remove from the oven, cover with foil and leave for 10 minutes.
Meanwhile make up the salad by mixing all the ingredients together.
Invert the cooked potato cake onto a plate and cut into wedges, serve hot or cold with a sprinkle of cheese and the tomato salad.