Cut the vanilla pod in half lengthwise and scrape out the pulp with the back of a knife. Peel the celery and cut into pieces. Melt butter in a pot. Sauté the celery for about 1 minute, then add milk. Add the scraped vanilla pod, pulp and a little salt and cook covered for about 25 minutes until soft.
Wash the scallops and dab dry. Season mussels with salt and pepper and turn them in flour. Tap off excess flour. Heat 1 tablespoon of oil in a frying pan. Fry the mussels for 2-3 minutes on each side
Wash the chervil and shake dry. Chop the leaves of 3 stems finely. Remove the vanilla pod and puree the celery. Stir in chopped chervil, season with salt and pepper. Arrange the mussels on the celery puree. Sprinkle with 2 tablespoons of olive oil and garnish with the remaining chervil