Add jalapeño slices to the shaker and muddle gently to release the oils.
Pour in the grapefruit juice, lime juice, and simple syrup. Fill the shaker with ice.
Shake well for 15-20 seconds until the outside of the shaker feels cold.
Strain the mixture into a tall glass filled with fresh ice. Top with club soda and stir gently.
Garnish with a grapefruit wedge and jalapeño slice.
Notes
Remove jalapeño seeds to control the heat level. Start with less jalapeño and add more to taste.Store fresh-squeezed grapefruit juice for up to 24 hours in the fridge.Make a sugar-free version by using a natural sweetener instead of simple syrup.Double strain through a fine mesh strainer to remove small jalapeño pieces if desired.Best served immediately while carbonation is fresh and ice is cold.