Prepare your mise en place.
Place the sugar in a heavy-bottomed saucepan over medium heat. Cook, stirring frequently, for about 5 minutes or until the sugar has caramelized. Add the water, vinegar, salt, cumin, fennel, coriander, and red pepper flakes. Bring to a boil, then remove from the heat and set aside for 20 minutes.
Place the mushrooms in a nonreactive container.