1. Place celery and onions in 4-1/2- to 5-1/2-quart slow cooker; top with corned beef brisket. Sprinkle contents of seasoning packet over brisket. Add 2 cups water and beer. Cover and cook on HIGH 6 to 7 hours or on LOW 9 to 10 hours or until brisket is fork-tender. (No stirring is necessary during cooking.)
2. Meanwhile, place cabbage, potatoes and carrots in 2-1/2 quart microwave-safe dish; add remaining 1/2 cup water. Cover; microwave on HIGH 15 to 18 minutes 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 minutes or until melted, stirring once. Whisk in mustard. Cover; microwave on HIGH 30 seconds. 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.
Test Kitchen Tips To cook on range-top, place corned beef in stockpot; sprinkle with contents of seasoning packet. Add celery, onions, 2 cups water and beer. Bring just to a simmer; do not boil. Cover tightly; simmer 2-1/2 to 3 hours or until brisket is fork-tender. Prepare vegetables and sauce as directed.