Heat the oil in a pan and sauté the onion and leek for 5 minutes to soften.
Meanwhile, cook the potatoes in a pan of lightly salted water until tender and then mash together with the milk and butter. Season to taste.
Add the minced chicken, celery and grated carrot and cook for about 5 minutes. Transfer to a food processor and chop for a few seconds to give the mixture a smooth texture, if desired. Return to the pan, add the ketchup, boiling stock and herbs.
Cover and cook for about 4 minutes.
Preheat the grill.
Divide the chicken between four 10cm /4 “ramekin dishes and top with the mashed potato. Dot the top with a little butter and place under the grill until golden.