For the salad, heat a frying pan over a medium heat. Add the coriander seeds and toast for 1 minute, shaking them occasionally, until aromatic. Tip them on to a plate and lightly crush using a knife blade.
Pour both vinegars into a small saucepan, add the sugar and cook over a medium heat for 2-3 minutes, or until the sugar is dissolved, stirring occasionally. Remove from the heat and stir in the coriander seeds, then leave until cold.
Put the mackerel fillets in a shallow, non-metallic dish, skin-side-down, and sprinkle with the salt. Pour over the vinegar mixture and top with three-quarters of the red onion. Cover with cling film and marinate in the refrigerator for 6 hours, or until pickled, turning the fish over halfway through.
Just before the mackerel is ready, fill a saucepan one-third full with water, place a steamer on top and bring to a simmer. Put the remaining red onion, the coriander sprigs and garlic in the water. Put the potatoes in the steamer, cover and cook for 12-18 minutes, or until just tender. Turn the heat off and steam them in the residual heat for a further 5 minutes, then leave to cool.
For the French dressing, whisk the Dijon mustard, vinegar and 2 tablespoons of water together in a small bowl. Whisk in both olive oils until thick and glossy. Season with salt and pepper.
For the mayonnaise, beat the egg yolks and French mustard together in a large bowl. Add a few drops of lemon juice and season with salt and pepper. Drizzle in the oil a little at a time, whisking continuously with a hand-held electric whisk, until thickened.
Whisk together 3 tablespoons of the French dressing, 2 tablespoons of the mayonnaise and the yoghurt in a bowl. Stir in the lime zest and juice and season with salt and pepper.
Cut the potatoes into 5mm/¼in slices and put them in a large salad bowl. Add the radishes and chopped coriander leaves, then drizzle with the dressing and toss gently. Scatter over the spring onion. Lift the mackerel out of the pickling liquor and put it on serving plates. Scatter over some pickled red onion and serve with the potato salad.