Clean your sink. Seriously. That's part of the recipe.
Grab a really big pot. Put the pork (and chicken if using) bones in the pot and add water until everything is submerged. This is not the water/chicken stock in the recipe ingredients. You will be chucking this water.
Bring to a boil. Not a raging boil but not a simmer either. Cook somewhere around 5-7 minutes. You will see a bunch of pretty grim looking goop (that's the scientific term) form on the surface. Getting rid of that is why you are doing this. OK - it's actually coagulated protein. Not goop. I like goop better.
While the bones are coming to a boil char the ginger and onion. People say use a gas burner on your stove. That is messy. Or they say use the oven. That takes way too long. I use my barbecue. You just toss it on and let fire do its thing. If I'm really jammed for time I've been known to use a blowtorch. Not saying that's a good idea though. But I run with scissors so... Dump the bones into the sink. Clean the pot. Put the pot next to the sink. And start cleaning the bones under running water. One by one. Any bits of additional goop stuck to the bones need to go. As you clean them drop them into the pot.
Toss the charred onion and ginger to the pot.
Add the chicken stock and enough water to cover everything by about an inch. Bring to a gentle simmer. At this point you want to pay attention. The goal is a nice clear broth. Don't let it boil hard. A nice, gentle simmer.
Simmer for 12 hours. Or more. For real. You aren't making stew. You are extracting maximum flavour. That takes time. The goal is for any meat on the bones to taste like nothing. All the flavour in the broth. That's the goal.
After you have convinced yourself you've extracted all the flavour remove the pot from the heat. Remove the bones (tongs and a slotted spoon work well for this. Strain into a clean bowl. I use a fine mesh strainer for broths.
You should wind up with about 16 cups of broth or enough for 8 bowls of hu tieu. This is a double batch. I like to freeze half for another day. If you want to make 8 cups total just half all the broth ingredients.