Rinse the cod fillets under the tap and pat dry with kitchen paper.
Peel and bash the garlic. Wash, deseed and thinly slice the chilli.
Squeeze the lemon. Finely chop the anchovy fillets and mix with the lemon juice and mustard. You want to end up with quite a thin sauce so, if necessary, add a tablespoon of hot water. Add the oregano and set aside.
Put the oil, garlic and chilli into a frying pan and cook over a medium heat for a couple of minutes.
Add the cod, cover the pan with a lid and cook the fish for 3 minutes on each side.
When the fish is ready, douse with the anchovy and mustard sauce and serve immediately.