Trim the rooty ends from your spring onions. Thinly slice the whites and light green. Save the darker green bits for the end.
Peel or scrub your carrot. Finely dice. Clean your celery. Finely dice. Peel and finely chop your garlic.
Get a large pot hot. Add a gloss of oil. Swirl in your veg with a pinch of salt and pepper. Lower heat. Sizzle till the veg is just softened.
Slice your spuds into 1 cm thick rounds. Fold them and the mustard through the softened veg. Sizzle a moment. Add your water or stock. Bring to the boil. Lower heat. Simmer for 10 mins or till the potatoes are tender right the way through.
Thinly slice the reserved spring onion greens. Strip the chervil leaves from its woody stalks. Add the spring onion and tarragon leaves to the soup right before serving.