Slice lengthwise down the sausage. Stuff with sliced onion and the chunk of mozzarella cheese. Layer thinly sliced potatoes on foil. Sprinkle potatoes with salt and pepper. Place sausage in middle and wrap together. Wrap this foil package in another foil, but have the seam on opposite side to prevent leakage. Place in hot coals of a dying fire for ½ an hour or place on heated BBQ grill for 10 - 15 minutes per slice.