Add agave nectar to glass along with bitters and one ice cube, stir. Add tequila and mezcal, another ice cube, a few more stirs
Add 1/3 cup agave nectar, 1/3 cup water, and zest of one orange (6-8 wide strips) into a small saucepan. Heat until barely simmering, then reduce heat to low and steep for 20 minutes. Remove from heat and strain, pressing zest against strainer to extract as much oils as possible. When cool, add 1/4 oz tequila blanco (or neutral grain spirit) and store in fridge.
Unknown to me, Phil Ward of Death & Co first composed the 'Oaxaca Old Fashioned' in '09...while this version is my own recipe and a modification of his Oaxaca, I certainly defer to him with credit as the original/authentic creator - http://www.kindredcocktails.com/cocktail/oaxaca-old-fashioned