Place potatoes in large bowl and toss with about 3 tablespoons extra-virgin olive oil, 2 tablespoons grill seasoning and rosemary. Grill potatoes 4 to 5 minutes on each side.
While the potatoes are cooking, combine oranges and red onion and dress with red wine vinegar and some extra-virgin olive oil.
Remove the potatoes from the grill to the dressed oranges and onions, toss to coat. When you grill potatoes they will be slightly drier than when you use other methods of cooking. By adding the potatoes to the dressing while they are hot, they really will soak in the dressing.
When ready to serve, add the arugula to the potatoes and toss to distribute.