Puree spinach, watercress, parsley, tarragon, and garlic with oil.
Add mayonaisse and mustard.  Blend until smooth.
Add salt, pepper, lemon juice to taste.
Serve over lettuce (iceburg offers a nice crunch and would have been common in the 1920s). 
This dressing is also good over cold, poached salmon.

Note: This dressing was created in the 1920's at the Palace Hotel in San Francisco in honor of William Archer's play, "The Green Goddess".