[I prefer galangal--even dried, to ginger, but ginger will do. S.C.] Place chicken breasts in pan with just enough cold water to cover. Bring to a boil over medium heat; immediately reduce heat so water barely ripples. Simmer uncovered, until done, about 12 minutes. Meat should be moist and opaque throughout; cook only to just beyond pink stage. Remove breasts and drain well. Cool to room temperature; remove and discard skin and bones. Shred meat into bite-sized pieces. Reserve. Combine slivered lemon, fish sauce, chile pepper, green onion and sugar; set aside. Combine coconut milk, chicken stock, lemon grass, mushrooms and galangal in a sauce pan over medium-high heat. Bring to a boil, reduce heat and simmer, uncovered, until lemon grass is tender, about 15 to 20 minutes. Add reserved chicken and lemon mixture and heat through, about 3 minutes. Serves 6 to 8 as a first course or 4 as a main course.