Snap ends and remove strings from pea pods. Melt butter in heavy large saucepan over medium heat. Saute pods and onions until onions are tender, about 5 minutes. Add broth and minced tarragon and bring to boil. Reduce heat and simmer 15 minutes. Mix in romaine and simmer 5 minutes. Puree soup as finely as possible in batches in processor using on/off turns, about 5 minutes. Strain through fine sieve into bowl. Stir in half cup creme fraiche. Season with salt, pepper and sugar or lemon juice if desired. Cool to room temperature. Cover and refrigerate until well chilled, at least 4 hours. (Can be prepared 1 day ahead.) Just before serving, ladle soup into individual bowls. Top with dollop of creme fraiche and fresh tarragon sprig. Garnish with fresh peas if desired.