Place the onion slices in a medium, stainless steel mixing bowl. Add the coriander, cumin, and red pepper flakes and toss to distribute.
In a small saucepan over medium heat, bring the vinegar and water to a boil. Add the sugar and stir until dissolved. Remove from the heat and pour over the onions. Mix well and season with a generous pinch of salt. Let cool to room temperature. Transfer to a jar and refrigerate for up to 1 week.