Courtesy the Washington Post https://www.washingtonpost.com/recipes/watermelon-hibiscus-sour/

MAKE AHEAD Watermelon-hibiscus syrup

• 2 cups (480 milliliters) fresh watermelon juice (see Notes)
• 1 1/2 cups (300 grams) granulated sugar
• 1/2 cup (20 grams) dried hibiscus flowers
• 1/2 teaspoon fine salt

For the drink

• Ice
• 1 ounce (30 milliliters) watermelon-hibiscus syrup
• 3/4 to 1 ounce (22 to 30 milliliters) fresh lime juice
• 1 1/2 ounces (45 milliliters) mezcal
• 1 dash Angostura bitters
• 1 lime wheel, for garnish (optional)

Directions Time Active: 20 mins | Total: 20 mins, plus 1 hour for chilling the syrup

• Make the watermelon-hibiscus syrup: In a saucepan over medium-high heat, combine the watermelon juice, sugar, hibiscus flowers and salt. Bring to a boil and boil for 30 seconds, then reduce the heat to low and gently simmer until the syrup is slightly reduced, about 5 minutes. Remove from the heat and let cool completely. Using a fine-mesh strainer, strain the syrup into a lidded container (discard the hibiscus flowers), and refrigerate until completely cold.
• Make the drink: Chill a cocktail glass. Fill a cocktail shaker with ice, then add the syrup, lime juice (to taste), mezcal and bitters. Shake hard, then strain into the prepared glass. Garnish with the lime wheel, if using, and serve.