Cut the fish into chunks and place in a food processor. Pulse a few times to break up the pieces but do not overprocess and turn into paste.
Transfer into a mixing bowl and combine with the rest of the ingredients (not the beaten egg). Stir thoroughly.
Make generous size round shapes pressing the mixture together ensuring it feels firm, not loose, then flatten gently to form fish cakes. Brush both sides with the beaten egg and coat with the remaining breadcrumbs.
In a large non-stick pan heat up about a tablespoon of the oil and fry the fish cakes over a medium heat for about 3 minutes on each side or until cooked through and nicely browned on the outside. Add a little more oil once you've turned them over. Remove from the heat and serve immediately.
I recommend using cod because of its light, flaky texture but you could use haddock, halibut, basa or sole if you prefer.
If using frozen fish defrost it first then gently squeeze out excess water and pat dry using paper towel.
Try not to overprocess the fish. It should still have some texture.
Use panko breadcrumbs rather than fine breadcrumbs as these can produce denser fish cakes.
Fry the fishcakes over a medium heat so they have enough time to cook through without browning too quickly. Thick fishcakes (as in photos) will need about 3.5 minutes on each side. Try not to overcook your fish cakes. Remove from the pan as soon as they are fully cooked.
Do not overcrowd the pan. You need to be able to comfortably turn the fish cakes over.
You can prepare the fish cakes up to 2 hours in advance. I don't recommend preparing them too far in advance as they contain raw onion which can affect the flavour of the fish if left uncooked for too long.
Best served immediately.
If using fish that hasn't been frozen before you can freeze the fish cakes for up to 3 months. Place raw fish cakes, without the egg and breadcrumb coating, in an airtight container and separate with parchment paper so they don't stick to one another and freeze for up to 3 months. To defrost the fish cakes remove from the container and spread on top of a plate. Brush with the beaten egg, coat in breadcrumbs and fry.