In a large mixing bowl, whisk the yolk together with the salt, pepper, mustard, and lemon juice. Add the olive oil in a thin stream, very slowly at first, whisking constantly. Keep whisking until all the olive oil is completely absorbed. Whisk in the boiling water to stabilize the mayonnaise. In another bowl mash together the avocado and the lime juice. Stir the avocado, onion, and parsley into the mayonnaise and cover. If not using right away, refrigerate for up to 2 days.
c.1997, M.S. Milliken & S. Feniger, all rights reserved