Rinse the dried shiitake mushroom then soak in a bowl of water (about 3 cups). Set aside.
Heat a frying pan in medium high heat, pour a little oil. Wash and clean chicken pieces; pat dry and season generously with salt and pepper. Raise heat/fire to high and fry chicken until browned then turn to sear the other side. Transfer seared chicken into a saucepan.
Drain shiitake mushroom (don't throw the water) and squeeze to get the extra water out; set aside 1/2 cup of soaking liquid. Pour the rest of
into the pan with seared chicken, make sure there's enough liquid to cover the chicken pieces. Cover and put the saucepan over hight heat and bring to a boil then lower heat to simmer.
While chicken is simmering, saute the onion (if using) and shiitake mushroom in the same pan that you used for frying the chicken pieces.
Cook until mushroom is soft and onion is translucent, add about 1/4 cup of reserve mushroom-water and bring to a boil while scraping the pan to get the yummy bits at the bottom then add this to the pot of simmering chicken; raise heat/fire to high and bring to a boil again then let simmer over low heat until chicken is tender.
Add the remaining reserved mushroom-water to dilute cream of mushroom soup and add it to the simmering chicken. Gently stir and bring to a boil again, test the seasoning, adjust if necessary then turn off heat. Serve with rice. Enjoy! :)