Combine chopped rhubarb, sugar, and water in a small saucepan. Add lime zest if using. Bring to a gentle simmer over medium heat, stirring to dissolve the sugar. Reduce heat and simmer 8–10 minutes until the rhubarb breaks down and the liquid is pink and fragrant.

Remove from heat and let the mixture cool slightly (5–10 minutes). Using a fine mesh sieve, strain the syrup into a heatproof container, pressing on solids to extract as much liquid as possible. Discard solids. Cool to room temperature, then refrigerate until chilled. Syrup will keep up to 7 days refrigerated.

Cocktail

Prepare glass

If rimming the glass, rub a lime wedge around the rim and dip into coarse salt or tajín. Fill a highball or Collins glass with ice.

Mix cocktail

In a shaker or mixing glass filled with ice, combine 2 oz tequila, 0.5 oz fresh lime juice, and 0.75 oz chilled rhubarb syrup. Shake or stir briefly (10–12 seconds) to chill and dilute slightly.

Strain the mixture into the prepared ice-filled glass. Top with 3 oz chilled grapefruit soda, gently stir once to combine.

Garnish and serve

Garnish with a lime wheel or grapefruit wedge and a rhubarb ribbon or thyme sprig if desired. Serve immediately.