Combine the chopped rhubarb, sugar, and water in a small saucepan. Add the grated ginger if using. Bring to a gentle simmer over medium heat, stirring until the sugar dissolves. Reduce heat to low and simmer for 8–10 minutes, until the rhubarb has broken down and the liquid is pink and fragrant.

Infuse and strain

Remove the pan from heat and stir in the basil leaves. Let steep for 10 minutes to infuse basil flavor. Strain the mixture through a fine mesh sieve into a heatproof container, pressing on solids to extract as much liquid as possible. Discard solids. Cool syrup to room temperature, then refrigerate until cold. Yields about 1 to 1 1/4 cups syrup; syrup keeps refrigerated in a sealed container up to 10 days.

Cocktail Assembly

Prepare glass

Fill a mule mug or highball glass with ice to chill.

Mix

In a cocktail shaker (no need to shake with ginger beer), combine 2 oz Tanqueray gin, 0.75 oz chilled rhubarb-basil syrup, and 0.5 oz fresh lime juice. Add a few ice cubes and give a quick hard shake (about 5 seconds) to chill and slightly dilute.

Finish

Strain the mixture into the prepared glass over fresh ice. Top with about 4 oz chilled ginger beer and gently stir once to combine.

Garnish and serve

Garnish with a basil sprig and a thin rhubarb ribbon or lime wheel. Clap the basil between your palms once before garnishing to release its aroma. Serve immediately.