Add the sugar, cucumber and Serrano pepper to a cocktail shaker. Use a muddler to press down and twist the mixture. Repeat this a few times until the cucumber releases juice and creates a syrup with the sugar.
Add the tequila, triple sec and lime juice.
Add ice to the shaker then close the top and shake vigorously.
Strain the drink into a glass filled with ice. Serve immediately.
Video
Notes
Strain the muddled cucumber and pepper mixture through a fine-mesh sieve for a smoother margarita, especially if you’re serving guests who prefer a cleaner, less pulpy drink.