Place brisket, seasoning packet, brown sugar, and bay leaves in a large Dutch oven or stockpot. Cover with water and bring to a boil. Simmer covered 2 hours.
Add potatoes and carrots and return to a boil. Reduce heat and simmer covered 30-40 minutes until tender.
Add cabbage and return to a boil. Reduce heat and simmer covered until cabbage is tender, about 15 minutes.
For Horseradish sauce strain and reserve 1-1/2 cups cooking juices.
Cut beef across the grain into slices. Serve with vegetables and Horseradish Sauce.
Horseradish Sauce: In a small saucepan melt butter over medium heat; stir in flour until smooth. Whisk in cooking juices, stir in sugar and horseradish. Bring to a boil constantly stirring.
Mustard Sauce: Mix all ingredients.