Celeriac remoulade with smoked fish

Method

1. Put the egg yolk in a large bowl with the mustard and some salt and black pepper. Little by little, add to the oils, beating as you go, either with a wooden spoon or an electric whisk. Don’t add more oil until the previous lot has been incorporated and the mayonnaise has thickened. Add 1 tablespoon of the vinegar and season with salt and black pepper.
2. Peel the celeriac and cut into matchsticks, dropping these into acidulated water (add the juice of 1/2 lemon to a bowl of water) as you go – this stops the celeriac discolouring. (You could do this on a mandolin or in a food processor on the shredding blade.) Halve and core the apples and thinly slice. Put immediately into a bowl, squeeze over the remaining lemon half and toss together.
3. Drain the celeriac well. Add to the mayonnaise, with the apple, dill and two-thirds of the horseradish and stir. Add the sugar and remaining vinegar. Taste, adding more mustard, salt, sugar or vinegar, if necessary.
4. Divide the remoulade and smoked fish between 8 plates. Sprinkle with the remaining horseradish and serve with lemon wedges.