Drain the canned tuna oil and pour into the bowl in which we want to prepare the salad.
Melt the butter, and add lemon juice to it. Mix, and this sauce is poured over the tuna. Then cut the onion into rounds (after it has been cleaned and washed, beforehand :)).
Mix lightly to break up the pieces of tuna and to intertwine the tastes, but not until everything is done :) I did not feel the need to add salt. But you can add salt and pepper.
It can be served with wholemeal, toast.