- With the rack in the middle position, preheat the oven to 180 °C (350 °F).
- Cut off the darke green foliage of the leeks. Cut the pale green and white part of the leeks into two pieces
- In a baking dish, spread the leek and celery pieces. Add the broth, thyme, bay leaf and clove. Season with salt and pepper. Cover with a sheet of aluminum foil. Bake for about 30 minutes. Remove the leek and celery and let cool. Strain the broth through a sieve.
- In a saucepan, combine 75 ml (1/3 cup) of the broth and the vinegar. Reduce to 45 ml (3 tbsp) of liquid. Let cool.
- In a bowl, whisk together the cooled vinegar reduction, egg yolk and mustard. Drizzle in the oil, whisking constantly. Season with salt and pepper. Place the vegetables and apples on serving plates and drizzle with the vinaigrette.