Hands on time: 5 minutes. Baking time: 20 to 30 minutes. Preheat oven to 350 degrees. Tear heavy-duty foil to make a 12-inch square; smear foil with oil. Place onion slice on half of foil. Trim fat from chop, spread both sides with mustard, place on onion, sprinkle with salt and pepper, top with rosemary if desired. Close foil to form triangle, roll edges and crimp firmly to seal, turning sealed edges upward. (A surprising amount of juices are generated during cooking.) Place in baking pan (lest those juices leak) and bake 20 to 30 minutes, depending on thickness of chop, or until chop is no longer pink in center. Transfer packet to dinner plate, open and turn chop with onion and juices onto plate. Serve, if desired, with potato or sweet potato baked in same oven. Per serving: 219 calories (41 percent from protein, 5 percent from carbohydrate, 54 percent from fat), 22 grams protein, 3 grams carbohydrate, 13 grams fat, 74 milligrams cholesterol, 325 milligrams sodium. Exchanges: 1/2 vegetable, 3 1/2 meat. From the Oregonian FOODday, 1/5/93. Posted by Stephen Ceideburg