In saucepan, bring 4 cups of water to boil, add rice noodles. Remove from heat and let stand 8-10 minutes until tender. Drain and rinse noodles in cool water for 30 seconds. Place noodles in serving bowls.
Bring 4 cups chicken stock, plus 1 cup water to boil.
Add mushrooms and chicken, turn heat to medium and cook for 3-4 minutes or until chicken is cooked through.
Remove from heat, add spinach, fish sauce and cilantro. Spoon over rice noodles.