Set oven to 450 F (no need to preheat). Spray baking sheet with nonstick cooking oil and add onions and ginger, spread out into a single layer. Place sheet in oven and bake until slightly charred.
Add beef shanks to pot and cover with water (about 6 cups). Then, using the sauté setting (adjusted to “more”), bring up to a boil. Once the water reaches a rolling boil, remove shanks and set aside. Discard water and wash all scum from the bottom of the pot.
Return the beef shanks in pot, followed by charred ginger and onions, fish sauce, sugar, star anise, cloves, cinnamon, fennel seed, coriander seed, salt, and top with 10 cups of water. Lock lid and cook for 30 minutes at high pressure.
Once cook time is complete, allow pressure to release naturally for at least 30 minutes (longer is better).
Pour the broth through a fine mesh strainer. Feel free to eat the beef shank, but we won’t be using it anymore for this recipe.
Prepare rice noodles according to instructions on box. I prefer (and recommend) the type you don't need to pre-cook.
Return broth to heat and bring back up to boil. You can do this in the pot using the sauté setting or on stovetop.
Place noodles in a heated bowl. Place equal amounts of uncooked steak into bowls. Do the same with the noodles.
Pour boiling broth over steak (this is what cooks the meat) and allow to sit for a minute or two.
Top each bowl with any combination of sliced onions, cilantro, basil, mint, and scallions. Serve immediately!