The onions will keep well in the refrigerator for up to 2 weeks but taste best when eaten within the first week. In the Yucatan, it's common that cebollas moradas are pickled with habañero pepper. Habañero is extremely spicy for those who are't used to eating spicy food, and should be handled with rubber gloves, so please be careful if you do decide to use them. Bitter (or Seville) oranges can be difficult to acquire, depending where you live. If you want a bit of orange flavor but can't find bitter oranges, rather than using all vinegar, we reccomend a mixture of ⅓ cup of juice from regular oranges, ⅓ cup of fresh lime juice and ⅔ cup of vinegar. If you find the flavor of onion to be a bit on the strong side, you can blanch the sliced onion for 15 seconds in boiling water, then drain and immediately rinse in cold water to stop them from cooking.