10 ounces rhubarb (about 3 medium stalks), cut into 1/2-inch slices
3/4 cup sugar
1 tablespoon fresh lemon juice
3 cups seltzer or 1 bottle (750 ml) dry sparkling Cava wine, chilled
Lemon and pink grapefruit slices, halved (optional)
In a medium saucepan, stir rhubarb, sugar and lemon juice over medium heat until sugar dissolves. Bring to a boil, then reduce heat to medium-low. Simmer, stirring occasionally, until rhubarb breaks down completely, about 6 minutes. Strain through a fine-mesh sieve, pressing solids to extract as much liquid as possible. Discard solids. Cover and refrigerate until cold, at least 2 hours and up to 1 day.
Divide syrup among 8 glasses. Top with seltzer or Cava. Garnish with lemon and grapefruit slices, if desired. Serve immediately.