In a medium bowl, stir together eggs, watercress, mayonnaise, mustard, salt, and pepper until combined.
Spread a thick, even layer of egg salad onto 3 bread slices. Top each with a remaining bread slice to make 3 whole sandwiches.
Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from each sandwich to create a 3-inch square. Cut each sandwich diagonally into 4 equal triangles. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for a few hours until serving time.
Notes
Egg salad can be made up to a day in advance, placed in a covered container, and refrigerated.
*We used Colman’s Original English Mustard.
Recipe by TeaTime Magazine at https://teatimemagazine.com/egg-and-watercress-tea-sandwiches-recipe/