I always use a slow cooker liner first to save on clean up
Put broth and chicken breasts into pot
Add onion, shiitake mushrooms, fish sauce, garlic and basil
Stir to submerge mushrooms
Cook on low for 7 - 8 hours
Remove chicken from pot, shred and return to pot
If your mushrooms are overpowering the soup (as mine were) you can remove some at this point. If they are in large pieces, I would suggest cutting them in to smaller pieces.
Add noodles and bean sprouts for last half hour or so of cooking.
Top with chopped scallions and a wedge of lime (optional)