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.