Combine gooseberries, onion, garlic, mustard, lemon juice, and 200ml vinegar in a pan.
Bring to a boil then simmer for 10 minutes until thick.
Add raisins, sugar, and remaining vinegar, then simmer for 30 minutes until syrupy.
Heat deep-frying oil to 190°C.
Mix mackerel, mashed potato, chilli, lime juice, and seasoning in a bowl.
Shape the mixture into 2cm diameter balls.
Prepare plates with flour, beaten eggs, and breadcrumbs.
Coat each fish ball in flour, then egg, and finally breadcrumbs.
Fry fishcakes for 4 minutes until golden-brown and drain on kitchen paper.
Mix chopped celery and watercress in a bowl.
Whisk olive oil, vinegar, honey, and seasoning to create the dressing.
Toss the salad with the dressing just before serving.
Serve fishcakes over a spoonful of chutney garnished with salad.