In a large heavy saucepan cook the fennel bulb and shallots in butter over mod. low heat,stirring occasionally, for 5 minutes, add the broth and bring to a boil. Simmer the mixture, covered, for 15 minutes, or until the fennel is just tender but not soft, and remove the pan from the heat. In a heatproof bowl whisk together the lemon juice and the yolks, whisk in 1 cup of the soup liquid, a little at a time, and add the mixture to the pan, whisking. Heat the soup over low heat, stirring constantly, to 175 F, on a deep-fat thermometer, remove pan from heat and stir in fennel leaves. Makes about 7 cups,serving 6 to 8.