1. Place celery and onions in 4-1/2- to 5-1/2-qt. slow cooker; top with corned beef brisket. Sprinkle contents of seasoning packet over brisket. Add 2 c. water and beer. Cover; cook on HIGH 6-7 hrs. or on LOW 9-10 hrs., or until brisket is fork-tender. (No stirring is necessary during cooking.)
2. Meanwhile, place cabbage, potatoes and carrots in 2-1/2 qt. microwave-safe dish; add remaining 1/2 c. water. Cover; microwave on HIGH 15-18 min. or until vegetables are tender, stirring or rearranging once. Drain; remove and discard cores from cabbage wedges. Add butter; season with salt and pepper, as desired. Toss to coat; keep warm.
3. Prepare Red Currant-Mustard Sauce. Place jelly in medium microwave-safe bowl. Cover; microwave on HIGH 1-1/2 to 2 min. or until melted, stirring once. Whisk in mustard. Cover; microwave on HIGH 30 sec. Stir; keep warm.
4. Remove brisket and carve diagonally across the grain into thin slices. Serve with vegetables and sauce. Garnish with parsley, if desired.
* Recipe Courtesy of The Beef Checkoff.