Cut the tuna in 4 slices.
Peel and cut the onion in slices.
In a frying pan heat up the butter so it melts.
Add the bacon pieces and onion and fry.
Heat up another frying pan and add some butter.
Fry the tuna quickly on each side.
Serve on a plate and add the bacon, onion and soy sauce on top of the tuna.