Place the tomatoes on a lightly oiled baking sheet. Drizzle over 2 tablespoons of the oil, then sprinkle each tomato half with a little sugar and vinegar. Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until browned and soft.
Meanwhile, heat a large, dry frying pan until hot. Rub the remaining oil over the mackerel and season well. Add the mackerel to the pan, skin-side down, and cook for 5 minutes until the skin is golden. Turn over and cook for a further 3 minutes, or until the fish is cooked through. Stir together the crème fraîche and horseradish sauce.
Arrange the rocket, roasted tomatoes and mackerel on serving plates and serve with spoonfuls of horseradish sauce on the side.