In a dry soup pot over medium-high heat, place the onion halves and ginger slices cut-side down. Sear until blackened and charred (about 3-4 minutes) to release aromatics.

Add the star anise and cinnamon stick to the pot, toasting briefly for 1 minute until fragrant.

Pour in the beef bone broth, fish sauce, and sugar. Bring to a boil, then reduce heat to low and simmer covered for 20 minutes to infuse the flavors.

While the broth simmers, prepare the rice noodles according to package instructions, rinse with cold water to stop cooking, drain well, and divide into two large serving bowls.

Strain the solid aromatics (onion, ginger, spices) out of the broth and discard them. Return the clear broth to the stove and bring to a rolling boil.

Arrange the raw, thinly sliced ribeye directly over the noodles in the bowls.

Ladle the boiling hot broth directly over the beef; the heat of the broth will cook the thin meat instantly to medium-rare.

Top with fresh herbs and serve immediately with lime wedges on the side.