Parboil beef bones in a pot of water for 5 minutes to remove impurities, then rinse and set aside.
Toast cinnamon sticks, star anise, coriander seeds, and cloves in a pan until fragrant, then set aside.
Char onion halves and ginger slices in a pan until fragrant, about 3-4 minutes.
Place toasted spices in a spice bag. Add beef bones, spice bag, charred onion and ginger, water, fish sauce, salt, and sugar to the Instant Pot.
Seal the Instant Pot and set to high pressure for 1 hour. After cooking, release pressure and remove the spice bag.
If using beef meatballs, add to the broth and set the Instant Pot to sauté mode, cooking until meatballs are heated through.
Assemble bowls with cooked rice noodles. Add thinly sliced raw beef to each bowl. Pour hot broth over the beef, along with the meatballs if using.
The heat will gently cook the slices of beef. Garnish with green onions, cilantro, and other toppings as desired.
