1. Prepare bouquet garni. Take one layer of leek and place parsley and thyme inside it, roll and tie securely with kitchen twine.
2. Slice the rest of the leek.
3. Set aside two handfuls of small cauliflower florets. In a large pot bring water to a boil. Add the rest of the cauliflower and cook for about 1/2 minute. Drain the water. Add potato cubes, sliced leek, bouquet garni and chicken broth. Bring to a boil, then reduce the heat, cover and gently simmer for 15-20 minutes until the vegetables are tender.
4. Meanwhile, in a bowl, whisk together egg yolk and 1/3 cup of heavy cream.
5. Prepare garnish. Heat olive oil in a medium skillet. Add the reserved cauliflower florets, season with salt and fry until brown and starting to soften. Add 1 tablespoon butter, cook for about 10 seconds and drain the florets on paper towel.
6. When vegetables are tender, puree the mixture in a food processor or blender until very smooth. Season with salt and pepper, add 1 tablespoon butter and the rest of heavy cream. Mix well.
7. Whisk in the cream and egg yolk mixture, reheat before serving. (Do not overheat the soup after adding this or it will curdle.)
8. Serve garnished with some of the reserved cauliflower florets and fresh chopped parsley.