1. Place the beef in a heavy 4- to 5-quart flameproof casserole or Dutch oven, and pour in enough water to cover the beef by about 2 inches. Bring to a boil over high heat, meanwhile skimming off any scum that rises to the surface.
2. Add the carrots, celery stalks, leek, onion, parsley sprigs, peppercorns and salt, then reduce the heat to its lowest point, partially cover the casserole, and simmer for 2½ to 3 hours, or until the meat shows no resistance when pierced with a fork. Transfer the meat to a heated plate and cover it with aluminum foil to keep it warm. Strain the cooking stock into a bowl and discard the vegetables. Skim as much of the surface fat from the stock as you can.
3. In a heavy 8- to 10-inch skillet, melt the butter over moderate heat, and when the foam subsides, add the flour. Stirring constantly, cook the mixture for 1 or 2 minutes. Do not let the flour brown. Slowly pour in 2 cups of the strained stock and then the cream.
4. Bring the sauce to a boil, beating constantly with a whisk until it is thick and smooth. Reduce the heat to low and simmer for 10 minutes. Add the chives and nutmeg and taste for seasoning. If the sauce is too thick for your taste, thin with a few tablespoons of the reserved stock.
5. To serve, carve the meat into thin slices and arrange them slightly overlapping on a large, heated platter. Spoon a few tablespoons of the sauce over the meat and pass the rest separately in a sauceboat.
