Combine chopped rhubarb, sugar, and water in a medium saucepan. Bring to a gentle simmer over medium heat, stirring occasionally, until the rhubarb breaks down and the sugar dissolves, about 8–10 minutes.

Macerate and strain

Remove from heat and let the mixture cool slightly (5–10 minutes). Using a fine-mesh sieve set over a bowl, press the solids with the back of a spoon to extract as much liquid and flavor as possible. Discard or reserve the solids for another use.

Finish shrub

Stir the hot rhubarb syrup into the apple cider vinegar, then add rosewater and 2 tablespoons fresh lemon juice. Taste and adjust: if too tart, add up to 1–2 teaspoons sugar; if too floral, reduce rosewater next time. Cool to room temperature, transfer to a clean jar, and refrigerate. The shrub will keep 2–3 weeks chilled.

Cocktail Assembly (Rhubarb Rose)

Fill a Collins or highball glass with ice.

Add 2 oz gin, 1/2 oz elderflower liqueur, 3/4 oz rhubarb-rose shrub, and 1/2 oz fresh lemon juice to a shaker or mixing glass. Stir briefly (10–15 seconds) with ice to chill and slightly dilute, or if you prefer, shake lightly and strain into the prepared glass.

Top with about 2.5 oz sparkling water (adjust to taste) and gently stir once to combine.

Garnish with a rhubarb ribbon or lemon wheel and an edible rose petal if desired. Serve immediately.