Make the simple syrup by whisking together ¼ cup sugar and ¼ cup water in a small sauce pot or bowl. Microwave for 45 seconds or until the sugar dissolves or simmer on the stove top until the sugar is dissolved. Store leftover simple syrup in the fridge for up to 2 weeks.
Place the strawberries and mint leaves in a whiskey glass. Muddle together until the strawberries are mostly broken down. If you don't have a cocktail muddler, you can muddle the strawberries and mint in a bowl with a potato masher.
Pour in the tequila, lemon juice, and simple syrup. Stir to combine. Add the crushed ice to about ¾ full. Top off with a splash of seltzer. Stir, taste for sweetness, and enjoy!
Making crushed ice: If you don't have crushed ice at home, you can make it by placing ice cubes in a plastic freezer bag. Use a hammer or meat tenderizer to smash the ice until mostly broken down.
Berries: This recipe also works well with raspberries and blackberries. If using frozen berries, they should be completely thawed before muddling.