Half-fill a very large saucepan with water and heat until lukewarm. Rinse the beef under cold water, drain, put into the pan and bring slowly to the boil. As soon as the water boils, lower the heat and skim the surface. Add the bouquet garni, onion and peppercorns. Partly cover and simmer for 1 hour.
While the beef is cooking, prepare the vegetables. Leave small carrots whole, but if you are using larger ones, cut them into 3 cm pieces. Cut the celery in half lengthways, then across into pieces about 5 cm long. Cut the leeks into 8 cm pieces. Halve or quarter the turnips and leave the onions whole.
When the beef has simmered for 1 hour, add the vegetables and continue cooking for 40–50 minutes, or until they are tender. Transfer the beef to a heated serving platter. Remove the vegetables with a slotted spoon and arrange them around the meat. Cover the platter loosely with foil and keep hot. Reserve 3 cups of the beef stock.
To make the horseradish sauce, melt the butter in a saucepan, remove from the heat, and stir in the flour and horseradish. Stir the reserved 3 cups of beef stock gradually into the horseradish mixture and bring to the boil, stirring continuously. Reduce the heat, season the sauce to taste with pepper, stir in the lemon juice and simmer, stirring frequently, for 10 minutes. Stir in the cream and pour the sauce into a heated sauceboat.
Serve the beef and vegetables immediately with the horseradish sauce and hot prepared mustard, if desired. Any leftover corned beef can be used to make delicious sandwiches.