Step 1
Place oxtail in a large heavy pot. Cover with water and bring to a boil over medium-high heat. Boil for 5 minutes. Remove from heat pour out water and rinse oxtail to remove excess fat and impurities. Place in a multi-functional pressure cooker (such as Instant Pot®).
Step 2
Use tongs to hold ginger over a burner flame to char slightly. Allow to cool then quarter. Char onion quarters in the same way.
Step 3
Combine cinnamon stick cardamom cloves star anise coriander and fennel in a shallow dry saute pan. Toast over medium-low heat for 2 minutes. Place spices in a tea filter bag and close.
Step 4
Add ginger onion and spice bag to the pressure cooker. Pour in enough water to cover. Add fish sauce green onions sugar and salt. Close and lock the lid. Seal the vent and select high pressure according to manufacturer's instructions; set timer for 120 minutes. Allow 10 to 15 minutes for pressure to build.
Step 5
Release pressure using the natural-release method according to manufacturer's instructions 10 to 40 minutes.
Step 6
Meanwhile place rice noodles in a large bowl and cover with hot water. Set aside until noodles soften about 15 minutes.
Step 7
Skim the broth if necessary. Taste and add more fish sauce sugar or salt. Discard ginger onions and spice bag. Remove oxtail from the broth and shred meat away from the bones and cartilage.
Step 8
Fill bowls with rice noodles and top with meat and broth.