Shepherds Pie

Name: Shepherd's Pie
Ingredients: One 5-pound boneless leg of lamb; 1 tablespoon beef base; 2 cups water; 1 1/2 cups port; 4 cloves garlic, sliced thin; 5 carrots, peeled, sliced in 1/2-inch pieces and cubed; 6 stalks celery peeled, sliced in 1/2-inch pieces and cubed; 5 parsnips, peeled, sliced in 1/2-inch pieces and cubed; 2 white onions, chopped; 2 tablespoons chopped thyme leaves; 2 tablespoons chopped rosemary leaves; 1/2 cup frozen green peas; Mashed potatoes, as needed, for topping; Beer, for serving (recommended: Guinness)