Put the egg yolks, mustard, white wine vinegar and salt into a food processor and blend very briefly to mix. Mix the olive oil and sunflower oil together. Turn on the machine again and very slowly trickle the oil through the funnel until you have a thick emulsion. Stop the machine, add the trimmed watercress, and blend once more until well mixed in. Spoon the mixture into a bowl, and if it is very thick, mix in a little warm water to loosen it a little.