To the food processor, add the mayonnaise, lemon zest and juice, mustard, salt, pepper and avocado. Process until well blended and smooth, then transfer to a resealable plastic bag. With scissors, snip off a bottom corner from the bag. Using the bag like a pastry bag, pipe the yolk mixture into the egg whites. Garnish with watercress. Serve immediately or cover loosely and chill until ready to serve.