Place the brisket in a large heavy pot and cover with cold salted water. Add parsley, onion, leek, celery, carrots, bay leaves and peppercorns. Bring to the boil and keep removing the foam that forms during the first 20 minutes of cooking. Reduce heat to. simmer and cook till the meat is very tender, aber 2 to 2 1/2 hours. Keep the meat warm.
Trim and peel the root vegetables and cut into small strips. Add to a small saucepan with a few tablespoons of the cooking liquid and simmer till the veg are almost soft. Set set to one side.
Measure out 500ml of the cooking liquid and place in a small saucepan. Bring to the boil and cook, uncovered, till reduced to about half. Reduce the heat and stir in the cream. Add horseradish, salt and pepper and add a dash of lemon juice, to taste.
Cut the meat into thick slices and place in a deep serving dish. Pour the sauce over the meat.
Heat butter in a frying pan and heat the veg strips, stirring to evenly coat them with butter. Arrange on top of the meat and serve.