Toast coriander seeds in a frying pan for 1 minute and lightly crush them.
Dissolve sugar in both vinegars over medium heat for 3 minutes, then stir in seeds and cool.
Marinate mackerel fillets with salt, the vinegar mixture, and most of the red onion for 6 hours.
Steam potatoes over water flavored with onion, coriander sprigs, and garlic for 18 minutes until tender.
Whisk Dijon mustard, vinegar, water, and both olive oils together to create a thick French dressing.
Beat egg yolks and mustard, then gradually whisk in oil and lemon juice to make mayonnaise.
Combine 3 tablespoons of French dressing, 2 tablespoons of mayonnaise, yoghurt, lime zest, and juice.
Slice potatoes and combine with radishes, chopped coriander, spring onions, and the creamy dressing.
Serve the mackerel topped with pickled onions alongside the potato salad.