Boil the potatoes until firm/tender, about 5 minutes. Preheat your barbecue to medium-high. Brush the centre of each piece of foil with 1 tsp. of the olive oil. Set a quarter of the onions, and 6 potato halves, in the centre of each piece of foil. Combine the remaining oil, lemon juice, rosemary and garlic in a small bowl. Drizzle over the onions and potatoes; season with salt and pepper. Pull up the corners of each piece of foil and crimp the tops to create sealed packages.
Set the potatoes on one side of the barbecue. Turn the heat off underneath them; leave the other side set to medium-high. Close the lid and cook the potatoes 20 to 25 minutes, or until tender. Transfer to a baking tray and keep warm at 200 degrees F in the oven.
Turn the other side of the barbecue back to medium-high. Season the steaks with salt and pepper. Lightly oil the grill. Grill the steaks to the desired doneness, allowing approximately 2 to 3 minutes per side for rare, and 3 to 4 minutes per side for medium-rare. Brush each steak, if desired, with barbecue sauce when almost done. Plate the steaks and serve a foil pack of the potatoes and onions alongside each one.