1) Heat half the olive oil in a saucepan and when hot add the onion and garlic. Cook for 5-10 minutes until softened. Add the cherry tomatoes, passata and chilli flakes, cover with a lid and simmer.
2) Meanwhile, heat the rest of the olive oil in a frying pan and fry the fish fillets as per the instructions on the packet (probably around 15-25 minutes depending on whether they are frozen).
3) Place each fillet on a plate and pour over the tomato sauce. Serves 4.