Clean the fish and let it simmer in plenty of boiling salted water with a good shot of vinegar for about 20 minutes. Meanwhile cook rice in salted water for 15-20 minutes. Clean, wash and cut the leek into rings. Blanch briefly in boiling water. Drain the peppers.
Cut cucumber into slices. Fillet mackerel and cut into small pieces. Mix all ingredients. Wash dill and chop finely. Mix mustard and vinegar. Season with salt and pepper. Whip oil into it, add slowly. Add dill. Add carefully to the prepared ingredients. Season to taste with some cucumber stock.
Season with salt and pepper. Whip oil into it, add slowly. Add dill. Add carefully to the prepared ingredients. Season to taste with some cucumber stock. Leave covered for at least 1/2 hour. Season to taste again before serving.