Trim fat from brisket. Place brisket in a large stockpot, and add water, pickling spice, and garlic. Bring to a boil; cover, reduce heat, and simmer 1 hour. Add Brussels sprouts and next 4 ingredients (Brussels sprouts through rutabaga) to pot, and bring to a boil. Cover and simmer 1 hour or until brisket is tender. Remove brisket and vegetables from pot; reserve 1 cup cooking liquid. Let brisket stand 5 minutes, and cut into thin slices. Set vegetables aside, and keep warm.
Combine flour, sugar, and mustard in a small saucepan; stir in vinegar until smooth. Gradually add 1 cup reserved cooking liquid, stirring with a whisk until blended. Bring to a boil over medium-high heat. Reduce heat; simmer, uncovered, 10 minutes. Remove from heat; add horseradish and margarine, and stir until blended. Serve horseradish sauce with brisket and vegetables.
Note: Even though this recipe has 40% calories from fat, a single serving has only 18.5 grams of fat. Add some New England brown bread and the calories from fat should amount to less than 30%.