Preparation In a small pot, combine the rhubarb and sugar with 3/4 cup water and bring to a boil. Reduce the heat to moderately low and simmer until slightly thickened and bright pink in color, about 20 minutes. Let the syrup cool then pour through a fine-mesh sieve set over a bowl. Press down gently and discard the solids. DO AHEAD: The rhubarb simple syrup can be made in advance and stored in an airtight container in the refrigerator for up to 1 week. In a cocktail shaker, combine 1 ounce of the rhubarb simple syrup with the gin and lemon juice. Fill the shaker with ice and shake vigorously until completely mixed, about 20 seconds. Strain into a highball glass and top with Champagne or soda water.
Preparation In a small pot, combine the rhubarb and sugar with 3/4 cup water and bring to a boil. Reduce the heat to moderately low and simmer until slightly thickened and bright pink in color, about 20 minutes. Let the syrup cool then pour through a fine-mesh sieve set over a bowl. Press down gently and discard the solids. DO AHEAD: The rhubarb simple syrup can be made in advance and stored in an airtight container in the refrigerator for up to 1 week. In a cocktail shaker, combine 1 ounce of the rhubarb simple syrup with the gin and lemon juice. Fill the shaker with ice and shake vigorously until completely mixed, about 20 seconds. Strain into a highball glass and top with Champagne or soda water.