Add the rhubarb, sugar, and water to a heavy-bottomed saucepan. Heat over medium-low heat, stirring occasionally, until the sugar dissolves and the liquid comes to a simmer.

Continue cooking, stirring gently for about 10-15 minutes until the rhubarb is completely cooked down and transforms into a loose jammy texture.

Strain through a fine-mesh strainer, stirring the rhubarb to release as much liquid as possible.

Cool to room temperature before making cocktails.

Pour the gin, rhubarb syrup, lemon juice, and aquafaba or egg white into a cocktail shaker.

Seal and shake vigorously without ice for 30 seconds.

Open the shaker, add ice, and shake again for 1 minute.

Strain into a Collins or highball glass.

Top with seltzer and garnish with a lemon wedge or twist. Drink immediately before the foam dissipates!