2 oz. Fresh grapefruit juice
1 Tbsp. Fresh lime juice
1/4 oz. Simple syrup
2 oz. Mescal or tequila
Moisten the rim of a tall glass with a lime or grapefruit wedge and rim with coarse salt. Pour in remaining ingredients and stir. Add ice and top with club soda.
From what I've read, this, not the margarita, is the preferred drink of Mexico. It's often made with grapefruit-flavored soda, like Jarritos (or Squirt), in which case you should leave out the grapefruit juice, simple syrup and club soda and just use as much grapefruit soda as you like. For my money, fresh juice, simple syrup and club soda is much preferred.