Place lamb shanks in an ungreased 13x9x2-inch baking pan. Sprinkle with salt and pepper. Bake, uncovered, in a 400 degree F oven for 30 minutes.
Remove from oven; reduce heat to 350 degree F. Carefully add broth to pan. Sprinkle lamb with onion, rosemary, garlic powder, and mustard. Cover pan with foil and bake for 2-1/2 to 3 hours or until very tender. Remove pan from oven. Transfer lamb shanks to a platter; cover and keep warm.
If you like, prepare gravy. Serve lamb with gravy and mint jelly, if you like. Makes 6 servings.
Pour the pan juices into a large measuring cup, scraping to loosen the crusty browned bits from the bottom of the pan. Skim fat from the drippings, reserving 2 tablespoons fat. In a saucepan, stir together the 2 tablespoons fat and 2 tablespoons all-purpose flour. Measure 1 cup drippings; stir into flour mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Season to taste with salt and pepper. Makes 1 cup.