Heat oil in large stockpot over medium heat until hot. Place beef Brisket in stockpot; brown evenly. Remove Brisket from stockpot; season with salt and pepper.
Add onions to stockpot; cook and stir 3 to 5 minutes or until onions are tender. Return Brisket, fat side up, to stockpot. Add beer. Bring to a boil. Reduce heat; cover tightly and simmer 2-3/4 to 3-1/4 hours or until Brisket is fork-tender.
Remove Brisket; keep warm. Skim fat from cooking liquid. Increase heat and bring cooking liquid to a boil. Reduce heat and cook 15 to 20 minutes or until mixture is thickened and reduced by half.
Meanwhile combine Mustard Sauce ingredients in small bowl. Stir in 2 tablespoons thickened cooking liquid.
Carve Brisket diagonally across the grain into thin slices. Pour remaining onion mixture over beef. Serve with Mustard Sauce.
Nutrition information per serving: 279 Calories; 81 kcal Calories from fat; 9 g Total Fat; 3 g Saturated Fat; 4 g Monounsaturated Fat; 62 mg Cholesterol; 728 mg Sodium; 1 g Total Carbohydrate; 0.8 g Dietary Fiber; 36 g Protein; 3.2 mg Iron; 5.2 mg NE Niacin; 0.4 mg Vitamin B6; 2.6 mcg Vitamin B12; 8.6 mg Zinc; 36.4 mcg Selenium;