in a small bowl mix the egg well with a dash of water or milk (as you like it).
add salt and pepper.
melt the fat in small/medium non- stick frying pan.
when it is hot-pour egg in and fry first side (prepare as you would a regular omlette).
now work fast: after turning omlette over, sprinkle the tuna over half the area,squirt some ketchup over the whole area then place slice (or more) of cheese (you can use any type you like or grate some from a chunk) over the tuna.
fold "empty" half over the tuna+cheese and press lightly.
You can flip it over again if you like it well done.
remove to plate and enjoy with a nice green salad and fresh bread.