Step 1Bring cabbage, shallots, carrots, turnips, potatoes, celery root and salt and pepper to taste to boil in water to cover. Reduce heat and simmer, covered, until vegetables are tender, 25 to 30 minutes.
Step 1Combine whipped cream, horseradish, mustard and mayonnaise in small bowl. Taste and adjust horseradish and mustard as needed. Refrigerate until chilled.
Step 1Put corned beef, peppercorns, mustard seeds, bay leaves and onions in large pot. Cover with hot water and bring to boil. Reduce heat and simmer, covered, until tender, about 3 hours. Skim surface occasionally and add hot water as needed to keep beef covered.
Step 2Transfer meat to cutting board, cover with foil and weigh down with heavy pot.
Step 3Strain cooking liquid, reserving stock and discarding everything else. If making ahead, refrigerate stock and corned beef separately. Before using, bring stock to boil, then reduce to simmer. Unwrap meat and warm in liquid for 5 minutes.
Step 4Transfer cabbage and vegetables to warm platter. Cut beef across grain into 1/4-inch-thick slices and arrange on platter. Spoon reserved stock over beef and vegetables to moisten. Serve hot with pitcher of warm stock and horseradish mustard cream.