Pour the pork stock into a saucepan and bring to the boil over a high heat. Reduce the heat to low and simmer. Meanwhile, dry-fry the whole onion in a frying pan over a medium-high heat until the outer layer is slightly charred all round. Add the charred onion to the simmering stock. Put the coriander seeds, cloves and Sichuan peppercorns in a spice bag or secure in a piece of muslin and add into the stock. Continue to simmer, covered, for 45 minutes.
Put the pork mince in a bowl and season with salt and pepper. Tip into a food processor and blend to a smooth paste. Put the pork back into the bowl, add the spring onion and mix until well combined. Using a teaspoon or melon baller, scoop up the paste and shape into chestnut-sized balls and arrange on a plate – they don’t need to be perfectly round. Repeat until all the pork paste is used. Cover with cling film and leave to one side.
Heat the oil in a large frying pan over a medium-high heat. Add the prawns and stir-fry for 3–4 minutes until they turn pink and are cooked through. Transfer to a plate and leave to one side.
Bring a saucepan of water to the 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 the boil, then add the pork balls and cook for 4–5 minutes until they are cooked through and float to the surface. Divide the pork balls into 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 and bring to a vigorous boil. Ladle the stock into the bowls, then sprinkle over the coriander and fried shallots. Serve immediately with the lime wedges and extra bean sprouts on the side.