Prepare barbecue or gas grill so that coals are white or flame is medium-high. Place butterflied leg of lamb over medium coals or grill 5 to 7 inches above heat source; grill lamb (turning once halfway through cooking time) until meat browns and reaches desired degree of doneness as follows: using a meat thermometer, cook to 145 degrees (medium rare, 33 to 53 minutes), 160 degrees (medium, 40 to 60 minutes) or 170 degrees (well done, over 60 minutes). Meanwhile combine orange juice, orange marmalade, and butter, ginger and dry mustard in a small saucepan. Cook over medium-low heat until marmalade melts, stirring occasionally. Combine lemon juice and cornstarch in a cup; stir into orange juice mixture and cook until thickened. Remove from heat; brush onto lamb during final 10 minutes of cooking. When basting is complete, reheat remaining orange juice mixture to boiling; remove from heat and serve as a sauce.
Ready in 1 hour