Peel half of one avocado. In a small bowl, mash avocado half. Add scallion, 1 teaspoon of lemon juice, 1/4 teaspoon of salt, and hot pepper sauce. Stir until well combined. Reserve.
In separate bowl, stir together mayonnaise, chili sauce, horseradish, Worcestershire sauce, and remaining teaspoon lemon juice. Season to taste with salt and white pepper. Reserve.
Peel remaining 1 1/2 avocados, cut into half-inch cubes, and place in a large bowl. Squeeze excess moisture from crabmeat. Add to cubed avocado and gently combine. Fold in mayonnaise.
Line bottoms of 6 chilled open champagne glasses or small glass serving dishes with watercress. Divide crab mixture evenly among glasses. Top each with a dollop of mashed avocado mixture.
Press egg yolks through fine mesh sieve; combine with parsley in a small bowl. Sprinkle yolk/parsley mixture evenly over each portion. Mimosas can be covered and refrigerated for up to 3 hours.
Adapted from In The Kennedy Style: Magical Evening in the Kennedy White House by Letitia Baldrige; menus and recipes by White House Chef Rene Verdon. (Doubleday, 1998)
Recipe from The Unofficial Mad Men Cookbook: Inside the Kitchens, Bars, and Restaurants of Mad Men by Judy Gelman and Peter Zheutlin (Smart Pop, 2011).