Preheat the grill to a medium-high heat and line a baking sheet with greaseproof paper.
Soak the sliced onion rings in white wine vinegar for 10 minutes.
Boil the potatoes in salted water for 15 minutes.
Grill the bacon for 10 minutes until it is crisp.
Skin and flake the smoked mackerel into large chunks.
Mix mustard, capers, vinegar, pepper, and olive oil to create the dressing.
Add half of the chopped dill to the dressing and half to the mackerel.
Incorporate the softened onions into the dressing.
Drain the potatoes, slice them in half, and toss them in the dressing.
Combine the potato mixture with the mackerel.
Cut the bacon into 4cm squares, add to the salad, and toss before serving.