Put the sugar, peppercorns, ginger, dill and one teaspoon of salt into a medium bowl then stir in the vinegar.
On a board, slice the top and bottom off the grapefruit then carefully cut away the skin and pith from the fruit. Cut between each segment and add the flesh to the bowl, reserving all the juices and pouring these into the bowl, too. Add the fennel and any fronds to the bowl. Season with a little black pepper and toss everything together (including the dill and grated ginger).
Rub the oil over the mackerel and season well. Pop the fillets, skin-side up, onto a baking sheet. Using a blowtorch, scorch the skin of the mackerel until charred and the fish is cooked through. Alternatively, preheat the grill to its highest setting and cook the fish for three to four minutes.
To make the dressing, mix the crème fraîche and ginger together in a bowl with a little seasoning.
To serve, spoon some dressing onto each plate, lay two mackerel fillets on top and half of the salad alongside each.