In a large frying pan, heat 1 tbsp of the oil over a medium to high heat and brown the chicken thighs on both sides. Set aside.
In a large saucepan, heat the other 1 tbsp vegetable oil over a low to medium heat and gently cook the onion, carrots and celery for about 10 minutes until softened. Add the chicken to the pan, along with around 700ml of the stock, enough to almost cover the chicken and vegetables. Bring to a simmer, then add the sweetcorn and thyme.
Simmer for 45 minutes until the chicken can be pulled apart easily, topping up with a little more stock if it reduces too much. Remove the chicken to a chopping board and shred it into thin strips, discarding the bone and skin. Add back to the soup and stir through the mustard.