Place brisket, contents of seasoning packet, brown sugar and bay leaves in a large Dutch oven or stockpot; cover with water. Bring to a boil. Reduce heat; simmer, covered, 2 hours., Add potatoes and carrots; return to a boil. Reduce heat; simmer, covered, just until beef and vegetables are tender, 30-40 minutes. (If pot is full, remove potatoes and carrots before adding cabbage; reheat before serving.), Add cabbage to pot; return to a boil. Reduce heat; simmer, covered, until cabbage is tender, about 15 minutes. Remove vegetables and corned beef; keep warm., For horseradish sauce, strain and reserve 1-1/2 cups cooking juices; skim fat from reserved juices. Discard remaining juices. In a small saucepan, melt butter over medium heat; stir in flour until smooth. Gradually whisk in 1 cup reserved juices. Stir in sugar, vinegar and horseradish; bring to a boil, stirring constantly. Cook and stir until thickened. If desired, thin with additional juices and season to taste with additional sugar, vinegar or horseradish. , Cut beef across the grain into slices. Serve with vegetables and sauce.