Use whatever fresh herbs are available to make your own special blend.
Melt butter in saucepan. Saute lettuce and watercress for 10 minutes. Do not brown. Add stock and herbs and season with salt and pepper. Cook 30 minutes, partially covered, over low heat. Remove from heat. Cool slightly. Combine cream and egg yolk in separate bowl. Pour ½ cup of hot soup into cream mixture, stir, and return mixture to soup. Heat but do not boil. Garnish with thinly sliced radishes or chopped chives. (Also good cold.)