wash your pork bones, place in a large pot and cover with cold water. Bring to a rolling boil.
Boil the bones on a high heat for 15 minutes. There should be a fair bit of scum on the top of the water. Drain the bones and rinse again with cold water. This part of the cooking process will get you a clearer stock when you've finished.
Put the clean bones back in the pot and cover with 3 litres of cold water. Bring back to a boil then turn the heat right down to a gentle simmer. Let it cook away for 45 minutes on a low heat. If there is any more scum floating, now is the time to skim it off!
Drop in the onion, daikon, garlic, peppercorns, coriander seeds, coriander roots and a teaspoon of salt. You don't want to over season as you will always be adding either fish sauce or soy to your finished dishes. Let it simmer for another hour and a half on low, you're looking for a few bubbles, not a boil. Strain through a sieve lined with cheesecloth into a clean bowl. Divide the stock into portions and freeze what you're not going to use straight away, it lasts a couple of months in the freezer.