Decorative Egg Platter
Peel the eggs and halve lengthwise. Rinse the sprouts, pat dry, and mix with the vinegar. Arrange the sprouts on a platter. Place the eggs cut side up on the platter.
Mix the egg yolk and the mustard in a blender. Gradually incorporate the oil while the motor is running. Begin with a few drops, then increase the amount to a thin stream until a thick sauce has formed.
Season the mayonnaise with salt, pepper, and lemon juice. Place the mayonnaise in a piping bag with a decorative tip. Pipe the mayonnaise onto the egg halves. Garnish with the watercress, and serve.