Wash purslane, drain and transfer to a bowl. Peel the onion and cut into fine rings.
Peel the egg and cut in half. Finely dice the egg white, put the egg yolks in the blender. Add the vinegar, oil, mustard sauce and mustard to the egg yolk. Mix until a creamy sauce is formed, season with salt and pepper. Pour the sauce over the salad and mix.