Melt butter in heavy large saucepan over medium heat. Add shallots; saute until soft, about 5 minutes. Add asparagus and coriander; stir 1 minute. Add vegetable broth; simmer until asparagus is tender (may take up to 30 minutes). Cool slightly.
Mix in the whipping cream and then cornstarch mixture and stir until thickened. (If it doesn't thicken within a few minutes, add a little more cornstarch and water mixture and if it is too thick, add a little water to thin it out.)
If your not using the Creme Fraiche, stir sour cream, lemon juice and lemon peel in a small bowl. Divide soup among bowls. Top with a dollop of Creme Fraiche or your sour cream mixture and serve.