TO PREPARE DRESSING: Combine mustard and egg in large mixing bowl, mix well and set aside. Place the cream and dried tarragon leaves in a small saucepan and bring to a boil over medium heat on the stove. Remove from heat and whisk hot cream into the mustard/yolk mixture. If using fresh tarragon, add it now. Slowly whisk in 3 tablespoons oil. Set dressing aside. Remove and discard any tough watercress stems and discolored leaves and place trimmed leaves in cold water. Remove and discard dark green outer leaves and root tip of butter lettuce. Separate lettuce leaves and wash with the watercress. Remove from water, dry well and set aside.
TO PREPARE POTATO SALAD: Cut potatoes into 1-inch pieces. Place in pot, cover with water, and place over high heat and cook until potatoes are soft. Drain potatoes, place in a mixing bowl, pour the vinegar over, cover and let sit for 5 minutes. Add the turkey to the potatoes in the bowl and mix well. To serve, add watercress and lettuce to the bowl. Mix well and mound on plates.