In a large pot, combine the beef broth, onion halves, sliced ginger, star anise, cinnamon stick, and smashed garlic. Bring to a boil, then reduce heat to low and simmer uncovered for 20 minutes to allow the flavors to develop.
While the broth simmers, soak the rice noodles in warm water for about 20 minutes or until softened. Drain and set aside.
Remove the onion, ginger, star anise, cinnamon, and garlic from the broth using a slotted spoon. Stir in soy sauce and fish sauce (if using). Taste and add salt as needed.
Bring the broth back to a simmer. Add the thinly sliced beef to the broth and cook just until the beef is no longer pink, about 1-2 minutes.
Divide the soaked rice noodles among serving bowls. Ladle the hot beef broth and cooked beef over the noodles.
Garnish with fresh bean sprouts, basil leaves, chopped green onions, lime wedges, and sliced jalapeños if desired. Serve immediately.