How to Make It
Heat grill to medium-high. Divide the potatoes between 2 large pieces of heavy-duty foil and, dividing evenly, toss with the vinegar, 1 tablespoon of the oil, and ½ teaspoon each salt and pepper. Fold the foil over the potatoes and seal the edges to form 2 packets. Place on grill and cook, turning once, until the potatoes are tender, 20 to 25 minutes.
Meanwhile, rub the halibut and onion with the remaining tablespoon of oil and season with ¼ teaspoon each salt and pepper. After the potatoes have cooked for 15 minutes, oil grill. Add the halibut and onion and cook until the halibut is opaque throughout, 3 to 5 minutes per side, and the onion is tender, 4 to 5 minutes per side.
In a small bowl, toss the arugula with the lemon juice. Top the halibut with the arugula and serve with the potatoes, onion, and lemon wedges.