Char onion and ginger, let the skin burn for 15 mins, until soften and fragrant, rotating them occasionally. Let cool and discard skins.
Place bones in a big pot, cover with water and bring to boil. Once water is boiling, cook for 5 mins, then rinse bones and empty pot.
Place bones back into the pot, fill up with water, as much as you want. Bring to boil and remove scums, then lower heat to medium, let simmer for half h.
Add in the rest of the ingredients and simmer bones for 2 h at least.
Adjust flavor to your taste (by adding fish sauce, sugar, salt. If needed, add in pho cube/pho pouch)