Boil new potatoes in salted water for 12 minutes until tender.
Add kale and cucumber to the potatoes for the final 3 minutes of cooking.
Drain the vegetables well and set them aside.
Whisk vinegar, rapeseed oil, and lemon juice in a jug and season with salt and pepper.
Season oatmeal with salt and pepper on a plate.
Brush herring fillets with mustard and coat thoroughly in the seasoned oatmeal.
Heat butter and a splash of oil in a frying pan over medium heat.
Fry fillets skin-side up for 2 minutes, then flip and fry for another 2 minutes until golden-brown.
Drain the cooked herring fillets on kitchen paper.
Toss potatoes, kale, cucumber, and spring onions with the dressing.
Serve the herring alongside the potato salad with lemon wedges.