Heat a non-stick frying pan over medium-high heat. Add 2 tsp oil and the chicken. Cook 3-4 minutes each side or until golden and cooked through. Remove the pan from the heat, cover and set aside to stand 10 minutes.
Meanwhile, heat the remaining oil in a large saucepan over high heat. Add the mushrooms and cook 4-5 minutes until they start to colour. Add the garlic, chilli and ginger, and cook, stirring, for 2 minutes or until fragrant. Add stock. bring to the boil. Reduce heat to low, cover and simmer for 10 minutes.
Place the noodles in a bowl. Cover with boiling water and stand 5 minutes until noodles soften then drain and keep warm. Shred the chicken and add to the stock with the combined lime juice, fish sauce and sugar, simmer 2-3 minutes until chicken is warmed through.
Add the spinach to the soup then remove from the heat. Pile the noodles into warmed soup bowls, ladle over the soup. Top with coriander and serve with lime wedges & extra chilli, if desired.