Drain the olives and pat dry. With a sharp knife, make three or four cuts in each olive from end to end, then cut each segment away from the stone as carefully as you can.
Put the meagre fillets into a steamer (if you do not have one, put them into a colander over a pan of boiling water and cover with a pan lid). Steam for about 10–12 minutes, depending on the thickness of the fish, until firm.
Meanwhile, bring a little water to the boil in a small pan, add the chopped celery and simmer for a minute or two, just to soften. Remove with a slotted spoon and drain on kitchen paper. Add the onion to the same water, and again, blanch for a few minutes to soften, lift out with a slotted spoon and drain on kitchen paper. Heat a little extra virgin olive oil in a pan. Add the blanched celery and onion and cook gently until they begin to colour, then add the olives and capers.
After a few minutes add the tomato purée, vinegar, sugar, and fish stock, stir, and bubble up until the liquid reduces and thickens a little.
Put one of garlic cloves on a chopping board and crush into a paste with the back of a large knife, put a handful of parsley leaves on top and chop finely.
Put the fish fillets on a serving plate, pour the sauce over them and sprinkle some parsley and garlic on top.