1. Chop the eggs up and put into a bowl with the mayonnaise and a pinch of salt and pepper, mix well.
2. Spread the mixture onto 2 slices of the bread and top with fresh watercress.
3. Finish with the remaining two slices, cut off the crusts and cut into triangles.
4. Serve immediately or store in the fridge in an airtight container until ready to use.

Variation: To spice it up a little: add chives and/or a pinch of cayenne pepper to the egg mayonnaise.