Cream of Lettuce Soup (Potage Cr?me de Laitue)

1 small onion, chopped
1/4 cup butter or margarine
2 large heads Boston lettuce or 2 small
    bunches romaine, finely chopped
1/4 cup all-purpose flour
3 cups water
1 tablespoon instant chicken bouillon
1 cup half-and-half
1/2 teaspoon salt
1/8 teaspoon pepper
Mint leaves or parsley

Cook and stir onion in butter in 3-quart saucepan over low heat until tender. Reserve 1 cup lettuce; stir remaining lettuce into onion. Cover and cook over low heat until lettuce wilts, about 5 minutes. Stir in flour; cook and stir 1 minute. Add water and bouillon. Heat to boiling, stirring constantly. Boil and stir 1 minute. Pour mixture into blender container. Cover and blend on high speed until smooth, about 30 seconds; pour into saucepan. Stir in reserved lettuce, half-and-half, salt and pepper. Heat just to boiling. Garnish with mint.