Place celery and onions in 4 1/2 to 5 1/2-quart slow cooker; top with corned beef brisket. Add corned beef seasoning packet, if included, and 3 1/2 cups water. Cover and cook on HIGH 7 hours, or on LOW 9 to 10 hours, or until brisket is fork-tender. Remove brisket; set aside. Discard cooking liquid and solids.
Place cabbage, potatoes and carrots in 2 1/2 quart microwave-safe casserole; add 1/2 cup water. Cover and microwave on HIGH 15 minutes, or until vegetables are tender, stirring once. Drain vegetables; add butter and salt and pepper, as desired; toss to coat.
Carve brisket diagonally across the grain into thin slices. Serve with vegetables and Red Currant-Mustard Sauce. Garnish with chopped parsley, if desired.
Place jelly in medium size microwave-safe bowl. Cover and microwave on HIGH 1 1/2 to 2 minutes or until smooth, stirring once.
Whisk in mustard. Cover and microwave on HIGH 30 seconds.
Servings: approximately 1 1/4 cups sauce