Carefully clean the celery, cut away the ends of the stalks, leaving beautiful leafy greens on the side. Cut the vegetables into slices. Stir juice of one lemon, both salts, mustards, half cream and pepper to a sauce. Chop celery leaves into small pieces except for a few. Add to the sauce. Quarter the apples, unpeeled, and remove the core. Cut apples into slices to begin with, then into sticks.
Shape immediately into the sauce so they don’t turn brown. Add the celery to the apples and mix well, seasoning with salt and pepper. Leave to infuse for at least 20 minutes, but preferably longer.
Lightly roast the hazelnut kernels in a dry frying pan without adding fat. Cool, then cut into flakes. Before serving, sprinkle over the celery salad and garnish with the reserved leaves.