Combine first 5 ingredients in a small bowl, stirring well.
Place lettuce leaves on 4 slices bread; top evenly with watercress. Layer turkey, bacon, and tomato evenly on watercress. Spread mayonnaise mixture evenly on remaining slices of bread; top sandwiches with bread.
Slice sandwiches in half diagonally, using a wooden pick to hold each half together.
Oxmoor House Healthy Eating Collection