For the potato salad, boil the new potatoes in a large saucepan of salted water until tender (about 12 minutes). Add the kale and cucumber to the pan three minutes before the potatoes are ready and boil until all of the ingredients are tender. Drain well and set aside.
For the dressing, whisk together the vinegar, rapeseed oil and lemon juice in a jug until well combined. Season, to taste, with salt and freshly ground black pepper. Set aside.
For the herring, sprinkle the oatmeal onto a plate and season with salt and freshly ground black pepper.
Brush both sides of each herring fillet with a little of the mustard, then roll in the seasoned oatmeal until completely covered.
Heat the butter and the oil in a large, heavy-based frying pan over a medium heat. Add the coated herring fillets to the pan, skin-side up (do this in batches if necessary). Gently press down on each of the herring fillets using a fish slice so that they stay flat. Fry for 1-2 minutes, or until the oats are golden-brown, then turn over and continue to fry on the other side for a further 1-2 minutes, or until the herrings are cooked through and the oats are golden-brown.
Remove the herring fillets from the pan using a slotted spoon and set aside to drain on kitchen paper.
Place the boiled potatoes, kale and cucumber into a serving bowl, stir in the spring onions, and drizzle over the dressing. Mix well and season, to taste, with salt and freshly ground black pepper.
To serve, divide the herring equally between two serving plates. Spoon the potato salad alongside and serve with a wedge of lemon.