In a small heavy saucepan melt 2 T butter. Add the scallions and garlic and cook, covered, over low heat for 5 minutes or until the scallions are very soft but not browned. Remove them to the top part of a blender. Add the coarsely mashed pulp of 1 large avocado. Sprinkle with 1/2 of the lemon juice and blend the mixture at high spped until it becomes a smooth puree. Set aside.
In a large saucepan melt the remaining butter. Add the flour and cook the mixture, stirring constantly, for 2 minutes without browning. Add the hot chicken stock and whisk until the soup becomes slightly thick and very smooth. Season the soup with salt and pepper and let it simmer, partially covered, for 30 to 40 minutes.
In a mixing bowl combine the egg yolks, Tabasco and cream and blend them well. Fold the avocado puree into the cream and yolk mixture. Pour the mixture into the soup and, stirring constantly, heat the soup through without letting it come to a boil. Correct the seasoning and keep the soup warm. Peel the remaining 1/2 avocado and cut it into tiny cubes. Sprinkle with remaining lemon juice and add them to the soup just before serving. Serve the soup garnished with chives and a spoonful of whipped cream.
NOTE: You may serve the soup cold and vary it by substituting Creme Fraiche for the whipped cream and fresh chervil for the chives.