For the horseradish cream
Whip the cream in a bowl until it holds firm peaks. Add the wine vinegar and whisk briefly to incorporate. Fold in the freshly grated horseradish and the crème fraiche and season with salt to taste.
For the Mackerel
Preheat the grill. Check the mackerel fillets for bones. Place the mackerel on to oiled tinfoil, skin side up, and brush the skin with mustard and then dip the fillet into the oats. Place the fillets under the grill for 5-6 minutes until cooked. Serve with a potato, broad bean and beetroot salad, horseradish cream, a lemon wedge and raw summer vegetables. Scatter chives on top.