How to Make It
In a large skillet, heat 1 tablespoon of the olive oil. Add the onion and cook over high heat until starting to brown, about 2 minutes. Stir in the apples and the remaining 2 tablespoons of olive oil. Spread the apples in an even layer and cook over moderate heat, without stirring, until tender and nicely browned on the bottom, about 3 minutes. Remove the skillet from the heat and stir in the mustard, chopped dill and lemon juice. Scrape the mixture into a bowl and season with salt and pepper.
Light a grill or preheat a grill pan. Drizzle the mackerel fillets with olive oil and season with salt and pepper. Grill the fillets skin side down over high heat until the skin is nicely charred and crisp, about 2 minutes. Turn the fillets and grill until just cooked through, about 2 minutes longer. Transfer the fillets to plates, skin side down. Top with the relish and serve.