Press pepper evenly onto all surfaces of beef roast. Heat oil in stockpot over medium heat until hot. Place pot roast in stockpot; brown evenly. Pour off drippings.
Add broth and 2 cups of leeks to stockpot; bring to boil. Reduce heat; cover tightly and simmer 2 to 3 hours or until roast is fork-tender.
Remove roast; keep warm. Skim fat from cooking liquid. Bring liquid to a boil and reduce for 5 minutes. Combine cooking liquid and remaining 1 cup leeks in stockpot. Reduce and simmer, uncovered, 6 to 8 minutes or until leeks are tender. Stir in mustard; remove from heat. Carve roast into slices. Serve with au jus.
Nutrition information per serving: 240 Calories; 90 kcal Calories from fat; 10 g Total Fat; 3 g Saturated Fat; 4 g Monounsaturated Fat; 77 mg Cholesterol; 289 mg Sodium; 7 g Total Carbohydrate; 0.9 g Dietary Fiber; 29 g Protein; 4.3 mg Iron; 3.9 mg NE Niacin; 0.4 mg Vitamin B6; 3.2 mcg Vitamin B12; 6.9 mg Zinc; 32.3 mcg Selenium; 4.2 mg Choline;