Heat the oven to 325°F (165°C). Pat the beef dry and season it with 1 teaspoon of the salt and 1/2 teaspoon of the pepper.
Heat the oil in a large Dutch oven over medium-high heat. Brown the beef well on all sides, then transfer it to a plate.
Reduce the heat to medium. Add the onion, carrots, parsnips, celery root, and garlic to the pot. Cook, stirring occasionally, until the vegetables begin to soften and pick up browned bits from the pan.
Add the bay leaves, allspice, sugar, mustard, vinegar, broth, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper. Scrape the bottom of the pot. Return the beef and any collected juices to the pot; the liquid should come about halfway up the meat.
Cover the Dutch oven and braise in the oven until the beef is fork-tender, about 2 to 2 1/2 hours. Turn the beef once halfway through cooking. Add a splash of broth if the pot looks dry.
Transfer the beef to a cutting board and loosely cover it with foil. Remove and discard the bay leaves and allspice berries from the pot.
Carefully blend the vegetables and cooking liquid until completely smooth with an immersion blender. Alternatively, blend in batches in a countertop blender, venting the lid carefully to release steam.
Stir in the cream and warm the sauce gently over low heat; do not let it boil. If you prefer a thicker sauce, stir in the cornstarch slurry and simmer gently until lightly thickened. Taste and adjust with additional salt, pepper, mustard, or vinegar as needed.
Slice the beef across the grain and serve with the creamy vegetable sauce. Garnish with parsley, and offer cranberry sauce and lemon wedges alongside. Bread dumplings are traditional, but egg noodles or mashed potatoes are easy alternatives.