Roasted Leg of Lamb

1 boneless leg of lamb (6-9 pounds)1 large white onion, sliced 1/4-inch thick 1/2 cup water 1/2 cup white vinegar1 teaspoon sugar1 teaspoon dry mustard1 teaspoon butter1 teaspoon salt1 teaspoon pepperHeat oven to 325 degrees.Lay onion slices in large roasting pan to create a single layer. Place the lamb on the onions.In a small bowl, combine water, vinegar, sugar, dry mustard, butter, salt and pepper; pour on or around lamb, and cook, 35-40 minutes per pound. Baste every 15-20 minutes, adding water as needed to prevent pan from drying out. Let lamb rest 20-25 minutes and slice. Serve with mint jelly.Serves eight to 10.Cook's note: Serve with plain mashed potatoes or boiled or roasted new potatoes as contrast with the very intense flavor of the lamb.Nutrition values per serving: 453 calories, 18 g fat (8 g saturated), 3 g carbohydrates, 1 g fiber, 65 g protein, 205 mg cholesterol, 389 mg sodium.