Finely chop the onion, garlic and parsley. Mix in the extra-virgin olive oil and vinegar, together with the salt and peppercorns.
Dunk the filleted white fish in the marinade and leave to soak for 2 hours to let it absorb all the flavours.
Grease a frying pan with a little extra-virgin olive oil and gently fry the fish until it is evenly cooked.
You can also cut the fish into cubes and serve as brochettes. Best served with a mixed leaf salad.