1.Place oxtail and beef bones into a large pot, then fill with enough water to cover the beef. Bring to a boil over high heat, then discard the water. Rinse the oxtail and beef bones until clean. Drain and set aside.
2.Turn on the sauté function of the Instant Pot on high. When the "hot" indicator is on and in a dry pot, add the onions and ginger. Allow the onions and ginger to char and flip to ensure this is done on both sides. Transfer to a plate.
3.Add the oxtail and sear until slightly browned. Meanwhile, toast the spices by placing a dry pan on medium heat. In one layer, add all the spices to the pan, shaking or stirring to ensure even toasting, then remove when you can smell the aroma of the spices. Place them into a filter bag and tie tightly.
4.Add 8 cups of water into the Instant Pot, followed by the beef bones, spice bag, apple, sugar and fish sauce.
5.Set the steam valve to the sealing position. Program to pressure cook on manual and high for 1 hour. When time is up, wait for 30 minutes to allow the pressure to naturally release.
6.Remove fat and residual scum from the beef broth with a fat skimmer and discard the spice bag. Strain the soup then let the broth cool down, refrigerate it, then remove fat before reheating the soup the next day.
7.To serve, boil rice noodles according to package instructions. Place a serving into a bowl, then top with green onions, cilantro, and onion slices. Add the broth, along with bean sprouts, basil, and a lime. Enjoy!