In a saute pan on medium low, toast star anise, cloves, coriander, fennel, and cardamon pods, and cinnamon for approximately 2-3 minutes until fragrant. Add immediately to crock pot.
Add oil to pan and cook onion and ginger until onion is browned and soft, approximately 5-7 minutes. Turn ginger over half way through. Add to crock pot.
Meanwhile, bring a large pot of water to a boil and add beef bones. Blanch for 10 minutes. Drain and rinse. Add to crock pot and top with 8 cups of water, sugar, and fish sauce. Cook on high for 4 hours or low on 8 hours.
With a sieve (or colander, but dependent upon colander size it may not capture all of the seeds) drain seeds, pods, etc. from broth from crock pot into another large pot them immediately return to crock pot.
For the last 30 minutes, turn to high (or add 30 minutes to the 4 hours) and add chopped bok choy. Put flank steak in freezer for about 10 minutes to make slicing easier. Slice steak as thin as possible.
In final 10 minutes add noodles and steak to crock pot (for rarer steak less time in the crock pot and for thinner noodles less time). Check noodles for desired done-ness at 5 minutes.
Serve immediately and top with your favorite toppings!