Wash and cut the potatoes in half. Season with salt, pepper, rosemary and olive oil. Toss to coat.
Dump onto a baking sheet and roast for 20-30 mins until fork tender.
To a large bowl add extra virgin olive oil, red wine vinegar, dijon mustard, and garlic powder. No salt or pepper because the potatoes are already well seasoned. Whisk to emulsify.
Add the warm potatoes in with the vinaigrette, along with arugula and red onion. Toss to combine.