Place the mackerel into a bowl with the crème fraiche, lemon juice, horseradish and a good pinch of salt. Using a wooden spoon beat until well mixed. Divide between four ramekins and place a little sprig of dill on top.
To make the salsa, place the beetroot in a bowl with the remaining ingredients, season to taste and mix well. Divide between four ramekins. To serve, place a ramekin of both on each serving plate and finish with a few crostini. Garnish with extra dill.