XO sauce is a complex dipping sauce that combines the heat of chillies and the umami savoriness of dried scallops, dried shrimp, and (sometimes) cured ham, along with garlic and alliums. The name derives from Cognac, where 'XO' ('Extra Old') denotes top quality. Chinese chefs, particularly in Hong Kong, compete to make the best XO sauce, often keeping their ingredients secret. The secret's out, so you can now make your own, then use it to enhance all sorts of Chinese dishes.