Place the beef brisket in the slow cooker. Add the beef broth, onion, ginger, star anise pods, cinnamon stick, fish sauce, and sugar.
Cover and cook on low for 8 hours until the brisket is very tender.
Remove the brisket from the slow cooker and slice or shred it. Discard the cinnamon stick and star anise pods.
Cook the rice noodles according to package directions in a large pot of boiling water, then drain.
Divide the noodles among four bowls. Top with brisket slices and ladle the hot broth over everything.
Serve with basil leaves, lime wedges, bean sprouts, and sliced jalapeƱos on the side.