This is one of the most common relishy garnishes you find in plancha and carbon taquerias in Mexico City and beyond, along with sliced radishes and cucumbers. It’s very simple, bright, spicy and crunchy—perfect sprinkled on a taco filled with practically any meat off the griddle or grill. It seems that using habanero chiles has become more popular these days than using manzanos (the way I encountered the preparation when I first lived in Mexico). Both are delicious, but habaneros have become more easily available. By the way, I make these cebollas with 1 habanero, but you’ll want to use 2 if you like really spicy food.
In a medium bowl, stir together all the ingredients. Let stand an hour or so, then taste and season with additional salt if you think necessary. The texture is best when served within a few hours, but the mixture can be refrigerated and served the following day.