Preheat the oven to 350°F. Grease a 3-quart casserole. Heat the oil in a large skillet, add the onion, and sauté until golden. Add the pork chops and brown on both sides. Arrange the sweet potato slices in the bottom of the casserole and top with the onions and pork chops. Dissolve the cornstarch in ½ cup of chicken stock. Heat the remaining 1 cup of chicken stock in the skillet with the thyme and pepper. Add the cornstarch mixture to the hot stock, stirring constantly until thick. Pour the sauce over the pork chops, cover, and bake for 1 hour, or until the pork chops and sweet potatoes are tender.