Cover eggs with water in a saucepan by 1 inch. Bring to a boil. Remove from heat. Let stand, covered, for 13 minutes.
Drain, and run under cold water. Peel, and slice into rounds.
Spread brioche slices generously with mayonnaise. Top each of 8 slices with 1 sliced egg and 1/4 cup watercress. Season with salt and pepper. Sandwich with remaining brioche.