Yuzu is a citrus fruit that tastes like a blend of lemon, lime and grapefruit. Fresh ones or the bottled juice are available online and may be at farmers markets or Japanese or Korean groceries. You can substitute a mix that’s 2 parts grapefruit juice to 1 part each lemon and lime juice to get a similar effect, and use the zest of any of the fruits for garnish.
Using a vegetable peeler or sharp paring knife, remove the yuzu zest in a long, winding strip. Spiral the zest up the sides of a highball glass, then fill the glass with ice.
Squeeze 2 tablespoons juice from the yuzu and pour into the glass. Add the simple syrup, then top off with sparkling water. Serve immediately with a stirrer for mixing.