Finely chop the garlic clove and mix with lemon juice and zest, salt and pepper. Sprinkle the fillets with it on both sides and cover and leave for 20 minutes.
Heat the olive oil, finely dice the onion and fry in it until translucent. Add tomato paste, tomato pieces, sugar and salt and stir. Let the sauce simmer over low heat for about 15 minutes.
Finely chop the parsley, stir into the sauce with the cream and remove from the heat.
Heat 1 tablespoon of oil in a pan and fry the fish fillets over medium heat for 2 - 3 minutes on each side. Serve with the tomato sauce.