Prep the corned beef and vegetables: 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.
Make the horseradish cream: Whisk all horseradish cream ingredients together in a small bowl to blend. Cover and refrigerate at least 2 hours.
Make the cabbage: 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.
Slice the corned beef and serve alongside the slow-cooked vegetables, cabbage and 1-1/2 cups horseradish cream.