Place the halved onion and ginger cut side down in a dry pot over medium-high heat until deeply charred and fragrant.
Add the star anise and cinnamon stick to the pot and toast for one minute to release the natural aromatic oils.
Pour in the beef bone broth and bring to a gentle boil, then reduce heat to low and simmer for 15 minutes to infuse the flavors.
While the broth simmers, cook the brown rice noodles according to package instructions, drain, and set aside.
Place the beef in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin pieces.
Stir the fish sauce and sea salt into the broth, then strain the liquid through a fine-mesh sieve to remove the solids.
Divide the cooked noodles between two bowls and arrange the raw beef slices on top in a single layer.
Pour the boiling hot broth directly over the beef to cook it instantly, then garnish with bean sprouts, cilantro, basil, chili, and a squeeze of lime.