Guava Ponche with Sweet Vermouth

Combine guavas, 3 cups water, hibiscus flowers, sugar, and cinnamon sticks in a large saucepan. Bring to a boil over medium-high. Reduce heat to medium-low, and simmer, uncovered, until guavas are slightly softened, about 15 minutes. Let cool to room temperature, about 1 hour. Proceed immediately to step 2, or, for a more intense guava flavor, transfer to a medium bowl, cover, and refrigerate overnight.

Pour guava mixture through a fine wire-mesh strainer set over a medium bowl. Using the back of a spoon, lightly mash guava mixture to release all of the juices; discard solids.

Stir together guava mixture, vermouth, apple, lemon juice, salt, and, if desired, Fernet in an ice-filled pitcher or bowl. Garnish with lemon slices. Serve in chilled cocktail glasses. Top each glass with a splash of Topo Chico, and garnish with mint.