Drain tuna & place in mixing bowl with onions, salt & pepper and half amount of cheese.
Crush 1/2 bag of potato chips & add to mixing bowl & mix together, with a little garlic powder and salt & pepper. If you can't get cheese & garlic chips, then cheese & onion will suffice, in which case, 1 clove of crushed garlic can be added.
Beat the egg & mix in with the tuna mixture.
Add a little milk if more binding required & place into ovenproof dish.
If you're adding fresh tomatoes, thinly slice tomato and layer with the tuna mix into the ovenproof dish.
Crush rest of crisps & mix with remaining grated cheese & sprinkle on top of tuna mix.
Bake in a medium oven for around 15-20 mins or until golden brown on top.