I know: pig’s knuckle soup. For some that doesn’t sound good, but don’t let it scare you. Trust me - Vietnamese pork noodle soup (Bánh canh) is one of the purest, simplest soups in Vietnamese Cuisine, with a clear, full-flavored pork stock and a wonderful mix of textures.
The literal translation of bánh canh is “cake soup,” which probably refers to the thick, chewy noodles made from tapioca starch or rice flour that are the star ingredient of this otherwise simple soup. Many variations of the soup exist, including one made with river crab, but the noodles are a constant in all of them. Vietnamese pork noodle soup (Bánh canh) are easy to make, but if you don’t want to make them, you can substitute fresh udon noodles. When ordering your pig’s knuckle , or trotter, ask for the front leg, which is more tender than the back.
Preheat the oven to 350 ° F. Place the onion on a small rimmed baking sheet or pie pan and roast for about 1 hour, until soft and beginning to ooze. Remove from the oven and let the onion cool until it can be handled. Peel the onion and cut in half.
While the onion is roasting, make the stock. To ensure the pot is large enough to blanch the bones without boiling over, put the bones in the pot and add water to cover by 1 inch. Then remove the bones and bring the water to a boil. When it is at a rolling boil, add the neck bones and knuckle slices, return the water to a boil and boil for 3 minutes. Drain the contents of the pot into a colander and rinse under cold running water. Rinse the pot and return the rinsed neck bones— but not the knuckle— to the pot.
Add the onion halves and 8 quarts fresh water to the pot and bring to a boil over high heat. Lower the heat so the liquid is at a simmer and skim off any scum that forms on the surface. Simmer for 2 ½ hours, skimming as needed to remove any scum that forms on the surface.
Add the pork knuckle and continue to simmer for about 30 minutes longer, until the knuckle is tender. The meat will still cling to the bone a bit but should be easy to chew. Cut off a small piece to test for doneness. Taste the stock and season to taste with fish sauce and/ or salt.
Bring the stock to a boil, add the noodles, and cook until they are tender yet still have some bite, according to package directions. If using udon noodles, cook only until warmed through, about 3 minutes. Meanwhile, to ready the garnishes, arrange the basil, bean sprouts, scallions, and cilantro on a platter and place on the table.
Drain the noodles and divide them evenly among warmed soup bowls. Top each serving of noodles with a piece of pork knuckle. Ladle the hot stock over the noodles and pork and serve immediately, accompanied with the platter of garnishes.