Peel, wash and dice the vegetables. Sauté the onions, celery and carrots in a little butter, add the potatoes and pour in enough stock until the vegetables are just covered. Cook for approx. 25 minutes until soft, then process into a chunky paste either with a potato masher or with a hand mixer with the addition of approx. 2 tablespoons of butter. Season to taste with salt, pepper and parsley.