First the onions are cut into fine rings and the garlic finely chopped. The herbs are also cut into small pieces.
Now the olive oil is heated in a small saucepan. Sweat the onions and garlic on a low heat and add the herbs to the oil. After a minute add the tuna and sweat everything for three minutes, then deglaze with white wine. Add enough liquid (broth or wine) to just cover the contents of the pan and add salt.
Let simmer for five minutes. Add capers and quartered olives, season with lemon juice and season with plenty of pepper - finally stir in a piece of cold butter.