Pour the sugar into a sauce pot then wash and dry your lemon. Zest your lemon over the sugar making sure to get as much of it as possible into the pot. Next, use your fingers to rub the zest into the sugar and get the oils incorporated.
Juice the lemon. Then add the juice to the sauce pot with the lemon-sugar and turn on the heat. Stir until the sugar has just dissolved and you have a syrup. Pour the syrup through a fine mesh strainer (to remove the zest) and into a new container to cool.
Oaxaca Old Fashioned:
First, measure and pour the lemon rich simple syrup, into a large mixing glass and add 3-4 dashes of molé (mexican chocolate) bitters. Stir with a small whisk or spoon until well mixed.
Add in the mezcal and tequila along with some full ice cubes and stir for 30 seconds.
In a separate glass, add a single, extra-large ice cube (if you have them) or new ice and pour in the chilled cocktail.
Using a peeler, preferably a Y-peeler for ease of use, shave off a piece of lemon rind and express it over the drink, skin side down. Rub the outside of the peel against the rim of the glass and serve.
Notes
If you don't want to make syrup at all, use agave.
If you want a darker syrup with caramel notes for the cocktail, turn up the heat and let the citrus syrup cook down to your desired color and flavor.
Because there is no mixer in this cocktail, I would suggest using tequilas and mezcals that you are content drinking neat for the best results.
I still recommend expressing a lemon peel over this cocktail, regardless of the garnish or syrup you use.
Garnish with a citrus slice (candied, fresh, or dried) or your expressed citrus peel.