Build over ice in a double rocks glass with a lemon- and nutmeg-scented sugar rim, stirring in Champagne.
1. I go with Irish whiskey because it's less tannic than bourbon or rye and plays nicely wjth others. 2. A sparkling wine with low sugar and lots of acid, to offset the lack of citric acid, works best, particularly if it's also delicious. 3. If lacking acid phosphate, other acidulents—lactart, verjus, etc—work fine, but the salts in phosphate work well to integrate flavors. You can sub 1/4 oz lemon juice. 4. Scenting a sugar rim can be as easy as grating the aromatics (in this case, nutmeg and lemon zest) into the sugar before rimming.
This is a pricy and indulgent take on one of the simplest drink orders possible.