Roughly chop the fish and pick over to check no skin or bones remain. place all of the fish cake ingredients - fish, flour, egg, cream, onion, parsley, dill, lemon zest and a little salt and pepper - in a food processor and pulse to mix well. You want to make sure large chunks of fish are broken up and everything is well mixed, but you also don't want it with no texture left at all.
Chill the mixture in the fridge for around 30 minutes, or a little longer, to help it firm up.
While the mixture is chilling, mix up the remoulade sauce. Mix together all the ingredients for the remoulade (mayonnaise, pickled cucumbers/cornichons, capers, parsley, lemon juice, mustard and curry powder) in a small bowl. Refrigerate until needed.
When ready to cook, warm some of the oil and butter in a small or medium non-stick skillet/frying pan over a medium heat. I'd suggest you cook in two or three batches so small/medium makes this easier.
With damp hands, take a heaped spoonful of the mixture (around the size of a golf ball or slightly bigger) and form into flattened ball. You can place on a plate temporarily as you form more, but I think it's probably easier (and helps avoid them sticking more) to simply place directly into the warm skillet as you form them. Continue forming to fill the skillet, without over crowding.
Cook for a few minutes on either side so that they become gently browned on both sides and the fish is cooked through. They should become a little more firm feeling when you gently press the middle of the fish cake (you want the fish cooked, but still juicy and not dried out). If needed, you can cook a little on the sides as well.
Remove the cooked fish cakes from the skillet and drain on kitchen paper then repeat in forming and frying the additional fish cakes in further batches.
Serve with the remoulade on the side.
Video
Notes
Dill can be a little bit of a dominant flavor - if you like dill, then you will enjoy these as written, but if not, then I would skip as you may find it too much.