Despite all the iterations found on menus across the United States, there is only one true Taiwanese pork belly bun, or gua bao. The classic fluffy steamed bun is filled with a rich, meaty, salty, savory slice of braised pork belly, but it's the three accoutrements served with it that are its true signature. To be considered the real thing, gua bao must always have pickled mustard greens, cilantro, and peanut powder.
Let's go over each of the fillings. First, there's the pork belly, which is cut into bun-sized slices and then red-braised, which means it's stewed in soy sauce and and rice wine that's flavored with five-spice powder and other aromatics. This style of pork is commonly found on Taiwanese tables, and is often served with sides that help cut the richness, such as lightly cooked vegetables and/or pickled vegetables like mustard greens.
That's why pickled mustard greens are an integral part of gua bao—it's the classic pairing in bite-sized portions. These pickles are made by lacto-fermenting a head of mustard greens (a process similar to making sauerkraut, kimchi, or Jewish deli pickles), which gives them a tangy flavor and turns them a dark, not entirely beautiful lizardy green color. The greens are then shredded or finely chopped.
To balance the fermented flavor and drab color of the mustard greens, fresh, bright green cilantro is added as well. Cilantro is a common topping in Taiwan, sprinkled on everything from soup to ice cream. When chopped, it has a way of sticking to whatever it's on, which is especially helpful in the case of a somewhat messy bun. To maintain its crisp bite, the cilantro is typically chopped with its stems.
Lastly, the bun is topped with peanut powder. Unlike the crushed peanuts commonly used in Southeast Asian cuisines, Taiwanese peanut powder is blended with rock sugar, and ground quite fine. You can make it yourself in a food processor or blender, swapping in brown sugar or raw sugar for rock sugar instead.
All combined, the Taiwanese pork belly bun is a messy, colorful, glorious combination of salty, sweet, pungent, and fresh flavors, with multiple textures to boot.