Drain the peach halves.
Peel and cut the carrot into bevelled slices. Bring the court-bouillon and white wine to a simmer and cook the carrot for 10 minutes. Remove from the heat, add the mackerel fillets and poach for 10 minutes, covered.
Rehydrate the gelatine in cold water. Drain the mackerel and strain the cooking stock, adjusting the seasoning. Bring it back to the boil and whisk in the gelatine.
Alternate the mackerel, the peaches cut into quarters, the carrot, the chopped dill leaves and the chopped onion in a bowl. Pour in the stock and leave to cool.
Store in the refrigerator overnight. Reserve a few cherries for finishing and mix the rest with a little of their juice to obtain a coulis.
Turn out the terrine just before serving and decorate with the cherries. Serve it with the coulis.