Bring the beef broth and rosemary sprig to a boil over high heat. Remove from heat.
Add sliced meat (ox tail or brisket, flank steak), reduce heat very low so it's just simmering gently in liquid with no bubbles visible on the surface of broth and cover pot tightly. Skim foam from top if necessary as needed. Simmer for about 2 hours or until meat is tender when prodded to taste.
Remove beef, set aside
discard rosemary sprig (if using).
Prepare garnishes and serving bowls: slice green onions thinly crosswise. Rinse bean sprouts in a colander under cold water. If you're not using noodle rice pads, soak desired amount of flattened wide flat rice vermicelli or glasswort (saltwater-harvested seaweed used for making Vietnamese banh chung) and drain them well. Divide into serving bowls. Top with beef slices
add cooked bean sprouts, green onions cut in half lengthwise if desired.
Optional: Add a scoop of shrimp tofu (to bao/dried rice paste) over the top and stir well. Dressings often include Thai fish sauce or soy sauce with lime wedges for squeezing on side, chili pepper flakes mixed into dressing.
Serve immediately while hot