Peel and finely chop the shallot. Slice the mushrooms. Slice the chilli into thin rings (flick out the seeds for a milder flavour).
Warm 1 tbsp oil in a pan over a medium heat. Fry the chicken breast fillets for 5 mins. Turn every so often till they’re browned all over. Add the shallot to the pan. Season. Fry for 5 mins till soft. Stir every so often.
Stir the chilli into the veg and chicken. Bash the lemon grass a few times with a rolling pin or pestle to bruise it. Add to the pan with the mushrooms.
Pour in 1.2 ltr boiling water. Cover and bring the soup to the boil. Turn down the heat. Simmer for 10 mins till the chicken is cooked through. While the soup simmers, peel the sweet potato. Use a spiralizer to make thin sweet potato noodles or use a vegetable peeler to peel it into ribbons.
Tip the peanuts into a dry frying pan over a medium heat. Toast for 2-3 mins till golden. Tip onto a board. Leave to cool for a few mins, then rub to flake off any skins. Roughly chop the peanuts and set aside.
Scoop the chicken out of the soup and transfer to a plate. Lift out the lemongrass and discard. Add the sweet potato noodles to the soup. Simmer for 5 mins till tender. Use two forks to shred the chicken.
Juice the lime. Stir the lime juice and 1 tbsp of the sugar into the soup. Add in the shredded chicken. Taste and adjust the seasoning. Ladle the soup into warm bowls. Top with the coriander leaves and toasted peanuts to serve.