In a large pot, combine the vegetable or chicken broth, fish sauce, soy sauce, cinnamon stick, star anise pod, and sliced ginger. Bring to a gentle simmer over medium heat.
Add the halved white mushrooms and your choice of cauliflower or broccoli to the simmering broth. Let them cook for about 5-7 minutes until tender yet crisp.
Prepare the thin rice noodles according to package instructions, typically soaking them in hot water for 5-10 minutes until soft. Drain and set aside.
Add the large shrimp to the pot and cook for an additional 3-4 minutes until they turn pink and opaque. Do not overcook.
Taste the broth and adjust seasoning with salt as needed. Remove the cinnamon stick and star anise pod before serving. In each bowl, place a portion of rice noodles, ladle the broth with shrimp and vegetables, and garnish with fresh cilantro.