In a 4-quart Dutch oven combine peas and broth. Bring to boiling; reduce heat. Simmer, covered, for 6 minutes. Add lettuce and green onions. Return to boiling; reduce heat. Simmer, covered, for 4 to 6 minutes more or until peas are tender. If desired, use a slotted spoon to remove 1/3 cup peas; reserve for garnish. Stir in tarragon. Cool slightly.
Transfer one-fourth of the soup to a blender or food processor. Cover and blend or process until nearly smooth. Repeat with remaining soup, blending or processing one-fourth at a time. Return all of the soup to Dutch oven. Stir in half-and-half to reach desired consistency; heat through. Do not boil. Add salt and pepper to taste.
Top each serving with a slice of French bread, some prosciutto, and some feta cheese. If desired, garnish each serving with reserved peas and a tarragon sprig.