Make the simple syrup (if you don’t have any): Combine equal parts sugar and hot water, stir to dissolve, and cool. Store extras in the fridge.
Prep the lime juice: Roll the limes on the counter to loosen the juice, then squeeze to get about 1/2 cup.
Strain out seeds.
Blend the base: In a blender, add lime juice, 1/4 cup simple syrup, 1/2 cup cold water, the mint leaves, and a pinch of salt. Blend for 10–15 seconds to break up the mint.
Add ice and blend: Add 4 cups of ice. Blend on high until thick and slushy, about 20–30 seconds.
If it’s too thin, add more ice; if too tart, add a little more syrup.
Taste and adjust: You’re aiming for bright lime, cool mint, and gentle sweetness. Adjust sweetness or acidity to your liking.
Pour and top: Spoon or pour into chilled glasses. Top each with a splash of sparkling water for lift and light fizz.
Stir gently to combine.
Garnish and serve: Add a mint sprig and a lime wheel. Serve immediately with straws or spoons.