Peel the carrots and apples and grate them coarsely; mix together in a bowl with a pinch of salt and half the lemon juice.

Make the mayonnaise as follows: break the egg into the blender (this method uses both yolk and white), add the mustard, the juice of the remaining half lemon, 4-6 drops Worcester sauce and a little salt and pepper.

Blend at high speed while gradually pouring the olive oil in a thin stream into the blender receptacle through the hole in its lid; the mayonnaise will become thick, pale and smooth.

Add a little more salt, pepper and lemon juice to taste and mix with the grated vegetables; chill briefly before serving in individual bowls, topping each serving with a little soured cream and a small sprig of fresh chervil.