Strip the trevise, wash it, wring it.
Wash the potatoes. Cook them with their skin in a pan of boiling salted water (about 20 minutes). Peel them and let them cool. Drizzle them with white wine, stirring gently to soak them well.
Wash the apples, cut them into thin slices. Remove the pips. Sprinkle them with lemon juice to prevent them from turning black.
In a bowl, mix the egg yolk and mustard. Gradually pour in the oil. When the mixture is homogeneous, add the fresh cream. Add salt and pepper. Wash and chop the chervil, add it to the sauce.
Arrange the treviso in a salad bowl (or in individual bowls). When the potatoes are very cold, mix them with the apples, arrange them on the radicchio, drizzle with the sauce.
