Transfer meat to a cutting board to cool slightly. Meanwhile, prepare sauce by pureeing cooking liquid and vegetables in the Dutch oven with an immersion blender (or cool 15 minutes and transfer to a blender). Season to taste.
TO SERVE IMMEDIATELY: Thinly slice meat across grain. Arrange on a platter and top with some sauce. Top with Celery-Horseradish Garnish if desired. Pass remaining sauce.
TO SERVE SOON: Thinly slice meat across grain. Arrange in a casserole dish or on an oven-safe platter. Top with some sauce and cover with foil. Keep warm in a 275° to 325° oven for up to 1 hour. Top with Celery-Horseradish Garnish if desired. Pass extra sauce, rewarmed if necessary.
TO SERVE LATER: Wrap cooled meat and transfer sauce to a covered bowl. Chill up to 2 days. To reheat, slice beef across grain; arrange in a casserole dish. Top with sauce (remove congealed fat first) and cover with foil. Warm in a 325° oven for 1 hour or until heated through. If you like, transfer beef to a platter and sauce to a pitcher. Top with Celery-Horseradish Garnish, if desired.