In large saucepan pour in the chicken stock. Add the lemongrass, 2 dried chilies, ginger, sliced onions, garlic, sugar, and fish sauce.
Bring to a boil then reduce to a simmer and simmer 30 minutes.
strain the soup through a fine mesh strainer discarding the solids. Return to pan and place pan back on stove. Add the zest and juice of the lime. Add the two red chilies and chicken. Bring to a simmer again and simmer 10 minutes.
Meanwhile prepare the noodles as directed on package. Letting them soak in boiling water 10 minutes.
Drain the noodles and add to soup. Stir in the cucumber and serve immediately.