Hibiscus-Lime Agua Fresca

Place the dried hibiscus flowers in a large heatproof bowl or pitcher with the ginger slices if using. Pour the boiling water over the flowers and add the sugar immediately while the water is still hot — the heat dissolves the sugar completely without requiring any stirring. Stir gently for 30 seconds to make sure the sugar is dissolved and the flowers are submerged. Cover tightly with a plate or plastic wrap and steep for 20 to 30 minutes. Do not over-steep: hibiscus becomes astringent and tannin-bitter if left too long, particularly if the water was very hot. The water will turn an extraordinary deep crimson-purple within the first 5 minutes as the anthocyanins leach from the dried petals — this color is not just beautiful but is also a practical indicator. When the liquid is so deeply colored that you cannot see through it even in a thin layer, the steep is complete. Strain through a fine-mesh sieve into a large pitcher, pressing gently on the flowers to extract all the liquid without forcing bitter tannins through.

Strain the hot hibiscus concentrate through a fine-mesh sieve into a large pitcher, pressing the spent flowers with the back of a spoon to extract maximum liquid. Discard the flowers. Add the cold water to the concentrate and stir — the mixture will bloom from dark crimson to a brilliant ruby-pink as it dilutes. Add the fresh lime juice and stir well. Taste: a perfect agua fresca is simultaneously refreshing and substantial — clearly tart from both the hibiscus and lime, clearly sweet, and with a clean floral depth underneath. Adjust sugar with a simple syrup if needed (equal parts sugar and water, stirred until dissolved), or add more lime if it tastes too flat. Chill in the refrigerator for at least 30 minutes before serving over large ice cubes in tall glasses. A slice of fresh lime and a few fresh mint leaves are the traditional garnish.