Make the filling: Bring 4 cups of water to boil. Add the dried hibiscus, turn the heat off and let it steep for 30 min. to an hour.steep for 30 minutes to an hour. Strain the mixture and save the tea to make agua fresca. Rinse the hibiscus flowers with clean water. Add the strained hibiscus flowers to a food processor fitted with the metal blade attachment and pulse until chopped.
In a pan set over medium heat, add oil and chopped onion, when it starts to brown add the hibiscus flowers. Mix constantly so the onion does’t burn. When the hibiscus flowers are starting to brown, add a generous pinch of salt and the serrano peppers. Let warm and set aside.
Make the guacamole: Cut the avocado in half, pit it, scoop out the flesh and add it to a bowl. Mash the flesh and mix in the salt, the garlic, and the lemon juice.
To serve: heat the corn tortillas, add a tablespoon or so of guacamole and then one or two table spoons of the hibiscus flower mix.