Cook the eggs 10 minutes in a
saucepan of simmering water to cover. Drain, cool under
cold running water, and peel. Roughly mash the eggs in a
bowl with the mayonnaise, mustard, and lemon juice.
Season with salt and pepper.
Spread 4 slices of toast with mayonnaise, if you
like. Arrange about 1/4 of the avocado slices on each.
Spread each with 1/4 of the egg salad and arrange
watercress sprigs on top. Top each sandwich with another
slice of toast and serve.