Melt sugar in a saucepan over medium heat until it forms a golden-brown caramel.
Blend all remaining jam ingredients except crème fraîche into a smooth purée.
Stir the purée into the caramel and simmer for 4-5 minutes until sticky.
Cool the mixture and then stir in the crème fraîche.
Mix mackerel, mashed potato, chilli, and curry powder together and season with salt, pepper, and lime juice.
Shape the mixture into 2cm diameter balls.
Prepare plates with flour, beaten egg, and breadcrumbs.
Coat each ball in flour, then egg, then breadcrumbs.
Heat deep-frying oil to 190°C.
Fry the cakes for 3-4 minutes until golden-brown and drain on kitchen paper.
Serve the croquettes warm with the chilli jam on the side.