Fire up your grill for medium-heat (350 F) grilling.
Tear off a large sheet of foil.
Place sausage, potatoes, bell pepper (if using) and onion in center of foil.
Drizzle with oi and sprinkle with salt, pepper, and rosemary.
Fold up the edges of the packet and seal.
Place onto grill and cook 30 minutes or so until the potatoes are tender.
Be careful opening the packets as hot steam will get trapped inside.