Fill a pressure cooker 2/3rds of the way with water and bring to a boil. Add the pig trotters to the boiling water and cook until you stop seeing red blood come out of the bones (about 10-15 minutes). The idea is to draw out as much of the gunk as possible into this first batch of water. Transfer the trotters to a bowl of cold water then repeat with the leg bones and chicken bones (you can use the same water).
Dump the now very dirty water down the drain and wash the pot. Scrub any dark brown scrum off all the bones and rinse them thoroughly. Return the cleaned bones to the pot and cover with water (the water should come up an inch above the top of the bones). Bring the pot to a boil and skim off any chunks or foam that floats to the surface. Keep doing this until you don't seen any more foam or scum floating up. This will take about 30 minutes.
While the bones are going, Heat 1/2" of oil in a pot over medium heat and add the head of garlic and ginger. Fry this until they are browned and shriveled up. Use a slotted or wire mesh to transfer the ginger and garlic to a bowl. Add the onions to the oil and fry these until caramelized and shriveled. Add the fried onions to the ginger and garlic and set aside.
Once the stock is scum-free, add the caramelized ginger, garlic, and onions, along with the white pepper to the stock. Affix the pressure cooker lid and cook on high pressure for 1 hour and 45 minutes. If you don't have a pressure cooker, cover with a lid and cook at a boil for 5-6 hours (you may need to check and add water periodically, the bones should be mostly covered in water).
Once the pressure is released use tongs to remove and discard all the bones. Remove any chunks of pork and set aside for another use. Strain the stock into a bowl and skim off any excess fat.