Bring a saucepan of water to the boil and cook the sweet potato until soft. Remove, drain and mash with butter, crème fraîche, salt and pepper.
Preheat the oven to 220 degrees C. Grease a baking tray.
In a large mixing bowl combine the mashed sweet potato with the fish, chilli, egg, herbs and spring onions. Mix well and season. At this stage you will need to gauge if the mixture is strong enough to form into cakes; add some flour if the mixture needs a little help. Form into 6 cakes using oiled hands and place on the prepared baking tray.
Bake for 15 minutes until the cakes develop a skin. Remove from the oven.
Heat a tablespoon olive oil in a frying pan and pan fry the cakes for about 3 minutes on each side until golden. Serve hot or cold.