Position oven racks in upper and lower thirds of oven. Heat oven to 350°F.
Place corned beef brisket in roasting pan; sprinkle garlic, contents of seasoning packet and peppercorns around brisket. Add water; cover tightly with aluminum foil. Braise in upper third of 350°F oven 3 to 3-1/2 hours or until brisket is fork-tender.
Meanwhile, place carrots, parsnips and cabbage on rimmed baking sheet. Drizzle with oil and toss gently to coat. Season with salt and ground pepper. Cover with aluminum foil. Roast in lower third of 350°F oven with brisket 55 minutes. Uncover; continue roasting 10 to 15 minutes or until vegetables are tender and begin to brown.
Remove brisket from roasting pan. Cover and refrigerate 1/2 of brisket (about 12 ounces), 3/4 cup each carrots and parsnips and 1/2 cup Lemon-Mustard Sauce for use in another recipe.
Carve remaining brisket diagonally across the grain into thin slices. Serve with remaining roasted vegetables and Lemon-Mustard Sauce.
Lemon-Mustard Sauce: Heat 1 tablespoon olive oil in small saucepan over medium heat. Add 2 tablespoons finely chopped shallot; cook and stir about 2 minutes or until tender. Remove from heat; cool 1 minute. Stir in 2/3 cup dairy sour cream, 1/3 cup Dijon-style mustard, 1 tablespoon lemon juice, 1 tablespoon chopped fresh dill and 1 teaspoon honey. Season with 1/4 teaspoon each salt and pepper. Cover; set aside.
Nutrition information per serving: 441 Calories; 266.4 kcal Calories from fat; 29.6 g Total Fat; 9.6 g Saturated Fat; 0.2 g Trans Fat; 1.4 g Polyunsaturated Fat; 14.6 g Monounsaturated Fat; 125 mg Cholesterol; 1561 mg Sodium; 18 g Total Carbohydrate; 5 g Dietary Fiber; 6.6 g Total Sugars; 24 g Protein; 0.6 g Added Sugars; 77 mg Calcium; 2.9 mg Iron; 628 mg Potassium; 4.8 IU Vitamin D; 0.3 mg Riboflavin; 4.5 mg Niacin; 0.5 mg Vitamin B6; 2 mcg Vitamin B12; 231 mg Phosphorus; 6 mg Zinc; 41.1 mcg Selenium; 95.4 mg Choline;