Preheat oven to 350oF. Arrange onion quarters, rounded side down, and ginger pieces on small baking sheet. Roast until the onions begin to soften, about 25 minutes. Using kitchen scissors, snip off the darf edges. Toast star anise, cardamom pods, and cinnamon stick in small skillet over medium high heat until aromatic and slightly darker in color, stirring occasionaly, about 3 minutes.
Bring broth, water and brisket to boil in large pot over high heat. Skim off foam that rises to the top. Add onions, ginger and spices to pot. Reduce heat to medium-low. Cover and gently simmer until brisket is tender, about 2 1/2 hours. Uncover; cool 1 hour. (Can be made 2 days ahead. Chill brisket in soup, uncovered.).
Discard any fat from surface of the soup; rewarm the soup slightly. Using tongs, tranfer brisket to cutting board. Slice thinly across the grain. Strain the soup into a large bowl; discard the solids in strainer. Return soup to same pot; boil 10 minutes. Add fish sauce and sugar; season to taste with salt and pepper. Reduce heat to low. Add brisket to soup and warm through.
Meanwhile prepare the garnishes.
Cook the rice noodles in medium pot of boiloing salted water until just tender; about 5 minutes.Drain.
Divide noodles among the 6 bowls. Add brisket from soup, then onions, basil and chilies. Pour hot soup over. Serve with the remaining garnishes.