Remove any tough outer leaves from lettuce heads and cut each into quarters. Remove tough stem end and core. Put quarters on chilled plates.
Combine sour cream, mayonnaise, vinegar and garlic in a food processor or blender. Add 2 tablespoons cold water and process until blended and smooth. Add parsley, salt and pepper; pulse briefly to mix. Cover and chill until ready to serve (can be made up to 2 days in advance).
To serve, spoon some dressing on each wedge and sprinkle with scallions, if desired. Serve remaining dressing on the side.