Put the bay leaf, pepper corns, slice of onion and salt in a pot.

Now fill the pot with water and add the Mahi Mahi.

Put the pot on a medium fire and cook uncovered till the water starts boiling, then lower the flame and keep simmering until the fish has cooked through.

Drain it and keep it to cool down.  Then clean it and keep aside.

Blanch the chopped celery in salty boiling water.

Chop the onion, put it in a frying pan and cover it with 1/2 glass of water and cook it until the water dries up.

Then add 3 tbsp of extra virgin olive oil and sauté the onions in it for a minute or two. Add the tomato concentrate, capers, olives and blanched celery with a little of its reserved cooking water and cook the sauce for 5 minutes.

Dissolve the sugar in the vinegar and add it to the sauce.

Cook it for a few minutes on a medium-high flame, then put the fire off and let it cool down.

Then cover the cooked Mahi Mahi with it.  Serve at room temperature.