Step 1

Spread 2 teaspoons Diamond Crystal kosher salt in an even layer on a small flat plate. Halve 1 medium lime. Cut 1 lime wedge from one half. Run the lime wedge over the rim of a Collins or Old-Fashioned glass to moisten. Invert the glass into the salt, turning it as needed, to coat the rim. Fill the glass with ice.

Step 2

Halve 1 medium red or pink grapefruit. Cut 1 grapefruit slice from one half, then juice the remaining pieces until you have 2 ounces. Place the grapefruit juice in a cocktail shaker. Juice the lime pieces until you have 1/2 ounce, then add to the shaker. Add 2 ounces blanco tequila and 1/4 ounce agave nectar. Fill the shaker halfway with ice, seal, and shake until the outside of the shaker is very frosty, about 20 seconds.

Step 3

Pour through a fine-mesh strainer into the glass. Top with 2 ounces sparkling water. Garnish with the grapefruit slice.