Place the chicken, the white parts of the green onions, garlic and cilantro stems in slow cooker. Fill the slow cooker 3/4 full with the chicken stock. Cover and set on high.
Place the onion and ginger on a baking sheet and broil for about 8-10 minutes, turning, until lightly charred. Add to the slow cooker along with the fish sauce. Place the star anise, cloves and cinnamon sticks in a small dry skillet. Toast over medium heat, shaking them, for about 3-4 minutes. Place in a square of cheesecloth along with the fennel seeds and bay leaf. Tie with kitchen twine and add to slow cooker. Cover and cook for 4 1/2 to 5 hours.
Life the chicken from the slow cooker with a slotted spoon. Strain the broth through a double layer of cheesecloth into a large heatproof bowl. Discard all of the solids and return the broth to the slow cooker and turn to low.
Cook the rice noodles per directions and rinse in cold water.
Shred the chicken and return it to the slow cooker. Divide the noodles among the individual bowls, ladle the hot broth and chicken over the top. Serve immediately with garnishes on the side.
There are a few extra steps to this recipe, like the roasting of the onions and the ginger and the toasting of the spices. You can skip those if you don't have time, but they do add so much flavor!