Peel the onion, clean the carrots and slice them all very thinly; there should be roughly equal amounts of each. Heat the butter in a pan and stew the onion and carrot gently for about 6 minutes. Add the peeled and thinly sliced potato. After 3 more minutes pour on the heated stock. Add salt and pepper and simmer, covered for 35 minutes, then leave to cool slightly. Puree in a blender with the parsley, then reheat, adding extra seasoning as required.