Pour the pork stock into a saucepan and bring to a boil over high heat. Reduce the heat to low and let it simmer. Meanwhile, dry-fry the whole onion in a frying pan over medium-high heat until the outer layer is slightly charred all around. Add the charred onion to the simmering stock. Place the coriander seeds, cloves, and Sichuan peppercorns in a spice bag or secure them in a piece of muslin, then add it to the stock. Continue to simmer, covered, for 45 minutes.
Put the pork mince in a bowl and season with salt and pepper. Transfer it to a food processor and blend until smooth. Return the pork to the bowl, add the spring onion, and mix until well combined. Using a teaspoon or melon baller, scoop up the mixture and shape it into balls the size of chestnuts, arranging them on a plate—perfectly round shapes are not necessary. Repeat until all the pork mixture is used. Cover with cling film and set aside.
Heat the oil in a large frying pan over medium-high heat. Add the prawns and stir-fry for 3–4 minutes, or until they turn pink and are cooked through. Transfer them to a plate and set aside.
Bring a saucepan of water to a boil and blanch the bean sprouts for about 20 seconds. Divide the warm, cooked noodles into deep soup bowls, then top with the blanched bean sprouts.
Bring the stock back to a boil, then add the pork balls and cook for 4–5 minutes, or until they are cooked through and float to the surface. Divide the pork balls among the bowls of noodles and add the cooked prawns.
Add the fish sauce, a generous pinch of salt, and the sugar to the pork stock, then bring it to a vigorous boil. Ladle the stock into the bowls, then sprinkle with coriander and fried shallots. Serve immediately with lime wedges and extra bean sprouts on the side.