Char the onions and ginger over an open flame or under a broiler until nicely blackened, about 5-10 minutes.

Blanch the beef bones in boiling water for about 5 minutes, then thoroughly drain and rinse under cold water.

In the slow cooker, combine charred onions, ginger, prepared bones, spices, and enough water to cover by an inch. Cook on low for 10-12 hours.

Strain the broth through a fine mesh sieve into a pot, discarding solids; skim off any fat if desired.

Cook rice noodles according to package instructions until al dente. Thinly slice raw beef and freeze briefly for easier slicing.

To serve, place noodles in a bowl, top with sliced beef, ladle hot broth over, and customize with herbs, bean sprouts, lime, and chili.