Place bone in chicken thighs in the basin of your slow cooker and cover with broth. Slice onion into quarters (you can leave the skin on, it adds color) and add that in along with the ginger piece, fish sauce, and brown sugar.
Place pho spices in a dry saute pan on the stove and turn heat on medium. Heat spices for 1-2 minutes until fragrant, moving them around with a spatula to avoid burning.
Pour warmed spices into the crock pot.
Cook on low for 8-10 hours or high for 5-6 hours.
20 minutes before you plan to serve, prepare rice noodles according to packaging directions.
Carefully remove the chicken to a side plate and pull the meat off of the bones. Strain the rest of the broth until all of the spices, onion, and ginger are removed. Taste broth and see if it needs salt added (fish sauce is pretty salty!).
Ladle hot broth into serving bowls and top with cooked rice noodles and a portion of shredded chicken (about ½ cup).
Add desired herbs to top (I used cilantro, green onions, and jalapenos) along with other flavoring agents like sriracha or additional fish sauce.