Place the corned beef in a large slow cooker and scatter the pickling spices on top. Layer the rutabaga, carrots, potatoes and leek in the cooker (in this order for even cooking). Add enough hot water (4 to 5 cups) to cover the meat by at least 1 inch, put the lid on the slow cooker and cook on high, 7 to 8 hours.
Remove the meat and vegetables from the slow cooker and keep warm. Put the cabbage in a microwave-safe dish with 2 cups cooking liquid from the slow cooker, cover and microwave until tender, 7 to 10 minutes. Meanwhile, boil another cup of cooking liquid in a small skillet until reduced by half, about 10 minutes. Mix with the horseradish and creme fraeche in a small bowl.
Slice the corned beef and serve with the slow-cooked vegetables, cabbage and sauce; reserve about a quarter each of the meat and vegetables and 1 1/2 cups cooking liquid for Corned Beef Hash.