Wash the hibiscus quickly enough to remove the dirt and put it in a pot with water until it boils.
Strain the hibiscus (don’t throw the water away; use it to make agua fresca).
In a pan, sauté the onion, tomato, and hibiscus with a little avocado oil, season with salt, and then add the sugar until the acid is removed from the hibiscus.
Spread the beans on a tostada, add the hibiscus mixture, and sprinkle the cheese on top.