Preheat oven to 375 degrees F. Heat the oil in a skillet set over medium-high heat. Season the pork chops with salt and pepper, brown on both sides, and then place in a 9" by 13" casserole dish. Place the apple, onion, and sage in a bowl and toss to combine. Spoon the mixture over the pork chops. Pour the chicken stock into the casserole. Cover and bake 60 minutes, or until the pork is tender.