1. Preheat the oven to 400°F.
2. Put the garlic and paprika into a mortar, add the salt, and pound to a smooth paste. Add a few drops of olive oil, then rub the flesh side of the mackerel fillets with the paste and set aside.
3. Make the vinaigrette. Put all the ingredients into a small bowl and whisk together with a fork. Season with salt and pepper to taste.
4. Line a baking sheet with parchment paper and brush lightly with olive oil. Lay the mackerel fillets skin side up on the paper and season the skin with sea salt. Roast for 8-10 minutes until the skin is crisp and the fish is cooked through. Remove from the oven and leave to rest.
5. Meanwhile, boil the potatoes in a large pan of salted water for about 15 minutes until tender, then drain. Return to the pan with 1 tablespoon of olive oil. Crush lightly with the back of a fork, then add the scallions and stir to combine. Season with salt, then add a couple of tablespoons of the vinaigrette.
6. Serve the crushed potatoes topped with the mackerel and a drizzle of the remaining vinaigrette.