[andygrif]

I would suggest leaving out the mayo (you don't need it with the fat that the melted cheese will release).

Mix up your tuna chunks and chopped onion, slap it on your panini. Grate the cheddar on the top, close up your panini.

Those George Forman jobbies are slanted correct? If so, when the cheese melts out the side it will drain off (bad!) Suggest you put a book or something thick under the front of the grill so that the cooking surface is level, then slap your rolls on the grill, close the lid and cook for about 3-4 mins, or until the ingredients are hot all the way through.

The thing about paninis (like ciabatta) is that they are best when heated through/toasted - and the tuna & cheese hot and melted will be better than a cold middle!