Preheat oven to 375°. In a Dutch oven, heat oil over medium heat. Add lamb and onion; cook and stir until meat is no longer pink. Stir in flour until blended. Gradually add broth. Bring to a boil; cook until thickened, 1-2 minutes, stirring to loosen browned bits from pan. Add carrots, leeks, 1 tablespoon parsley, rosemary, salt, pepper and thyme. , Spoon into a greased 13x9-in. or 3-qt. baking dish. Cover with potato slices; brush with 2 tablespoons melted butter. Bake for 1 hour; brush the potatoes with remaining butter. Return to oven; bake until the meat is tender and potatoes are golden, 30 minutes to 1 hour longer. Cool briefly; sprinkle with remaining 1 tablespoon parsley., Freeze option: Before adding parsley, cover and freeze the baked, cooled casserole.To use, partially thaw in refrigerator overnight. Remove from the refrigerator 30 minutes before baking. Preheat oven to 350°. Reheat, covered, until a thermometer reads 165°, about 1 hour. Sprinkle with parsley.