Trim the roots from the salad onions. Thinly slice the white and light green parts, saving the darker green bits for later. Peel or scrub the carrot. Finely dice it. Finely dice the celery. Peel and finely chop the garlic.
Warm a large pan over a medium heat. Add 1 tbsp oil, the chopped veg and garlic with a pinch of salt and pepper. Lower the heat and sizzle for 5 mins till the veg is just softened.
While the veg cook, slice the potatoes into 1cm-thick rounds. Add them and 1 tbsp Dijon mustard to the softened veg. Sizzle for 1 min. Crumble in the stock cube and add 700ml boiling water. Bring to the boil, then lower the heat and simmer for 10 mins, or till the potatoes are tender right the way through.
Thinly slice the reserved green salad onion tops. Roughly chop the chervil leaves and stalks. Ladle the soup into bowls and add the salad onion greens and chervil to the soup just before serving.