Preparation of the soup:
Peel the potatoes and cut them into small cubes. Wash the leeks and slice them into thin rings. Finely chop the sweet onions.
Melt a knob of butter in a large pot and sauté the onions, leeks, and potatoes over low heat until they are soft but not browned.
Add enough chicken broth to just cover the vegetables. Season with pepper and salt.
Let the soup simmer gently until the potatoes are cooked, about 15-20 minutes.
Puree the soup with an immersion blender until smooth and strain if desired for an extra fine texture.
Preparation of the watercress cream:
Whip the cream to soft peaks.
Blanch the watercress briefly in boiling water, drain, and shock in ice water. Puree the watercress into a smooth coulis.
Gently fold the watercress coulis into the whipped cream. Add the grated Comté and stir until smooth cream forms.
Serving:
Ladle the soup into bowls.
Add a spoonful of watercress cream in the center of each bowl.
Garnish with a few hazelnuts, pieces of candied chestnut, and edible flowers.
Tips
For an extra creamy soup you can add a dash of cream to the soup just before serving. Use fresh chicken broth for the best taste. The watercress cream can be made in advance and stored in the refrigerator until use.