Place brisket in a large Dutch oven; cover with water. Add brown sugar, bay leaves and spice packet enclosed with brisket. Bring to a boil. Reduce heat; cover and simmer for 2 hours.
Add potatoes and carrots. Return to boiling. Reduce heat; cover and simmer 30-40 minutes or until meat and vegetables are just tender. Remove potatoes and carrots and keep warm. Add cabbage; cover and cook about 15 minutes or until tender. Discard bay leaves. Remove cabbage and meat. Let meat stand a few minutes, then slice across the grain. Serve with Horseradish Sauce or Sour cream mustard sauce.
Horseradish Sauce: In a small saucepan, melt butter. Blend in flour. Add 1 cup cooking liquid; stir until smooth. Add vinegar, sugar and horseradish. Cook and stir over medium heat until thickened and bubbly. Adjust seasoning with additional vinegar, sugar or horseradish if needed. Thin sauce if necessary with the remaining cooking liquid.
Sour Cream and Mustard Sauce: Combine all ingredients in a small bowl. Mix until well blended.
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