Select the youngest, freshest vegetables that you can find. Wash, scrape or cut them to the sizes specified in the ingredient list. Then, except for the spinach, place all of the vegetables in a 2-3 quart pot, cover with cold water and add the salt. Boil uncovered for 5 minutes or until the vegetables are tender. Add spinach and cook another 5 minutes. Remove the pan from the heat and strain the liquid through a fine sieve into a bowl. Set the vegetable stock and the vegetables aside in separate bowls.
Melt 2 tablespoons of butter in the pan over moderate heat. Remove from the heat and stir in the flour. Slowly pour in the hot vegetable stock, beating vigorously with a wire whisk and then beat in the milk. In a small bowl, combine the egg yolk and cream. Whisk in 1 cup of the hot soup, 2 tablespoons at a time. Now reverse the process and slowly whisk the warmed egg yolk and cream mixture back into the soup. Add the reserved vegetables to the soup and bring to a simmer. As soon as it comes almost to a boil, reduce the heat, add the cooked shrimp and simmer uncovered over low heat for 3 to 5 minutes or until the shrimp and vegetables are heated through. Taste and season the soup with white pepper as well as additional salt if necessary. Pour into a soup tureen and sprinkle with finely chopped parsley or dill.