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 1/2 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.