Short stir (a 10 count), strain into a chilled coupe.
The cognac should be an older style with some sweetness, color and boise. For the sherry, use something darker, not a Chipiona. I used Valdespino - the one with the gorgeous orange label.
Started out as a Toronto riff. I love a rye/cognac split base, and the sherry was at hand. It just took the realization that too much dilution makes the Fernet overly medicinal.