Place the sliced yellow onion and fresh ginger in a dry pot over medium-high heat, charring them until fragrant and slightly blackened on the edges.
Add the beef bone broth, star anise, cinnamon stick, fish sauce, and coconut sugar to the pot and bring to a boil.
Reduce heat and simmer the broth for 20 minutes to allow the aromatics to infuse, then season with sea salt and black pepper.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package directions, then drain and set aside.
Thinly slice the raw flank steak against the grain into paper-thin pieces to ensure they cook quickly in the broth.
Divide the cooked noodles into a large serving bowl and arrange the raw beef slices on top of the noodles.
Pour the boiling hot broth directly over the beef to cook it instantly, then garnish with bean sprouts, cilantro, Thai basil, and a squeeze of lime.