Cook the potatoes in boiling salted water to cover for about 15 minutes or until tender.

When the potatoes are cooked, leave them to cool slightly in the water, then transfer both potatoes and water to a blender and blend until smooth. (If you do not have a blender, pass them through a sieve.) Return the puree to the rinsed-out pan.

Melt the margarine in a large frying-pan, add the onion and garlic, if using, and fry over moderate heat until beginning to soften. Add the remaining vegetables to the pan and cook, stirring occasionally, for about 10 minutes, until just beginning to colour.

Mix the vegetables with the potato puree in the saucepan, then stir in the stock, milk, thyme and celery salt. Add pepper to taste.

Bring to the boil, lower the heat and simmer gently for about 15 minutes or until the vegetables are just soft. Stir in the cheese, reserving 2 tablespoons, and simmer for a further 2 - 3 minutes. Taste and adjust seasoning.

Pour into a warmed soup tureen. Sprinkle with the chopped parsley and the remaining cheese and serve at once.