Slice potatoes in half, drizzle with olive oil and season with salt and pepper.
In a small bowl, combine the onion slices with sherry vinegar and salt. Chill for at least 1 hour.
Whisk together the dressing ingredients in a large bowl. Set aside.
Heat a grill pan* to high heat and brush with oil.
Place the potatoes, cut side down and let them cook without moving them until nice char marks start to form (flip one to check). After one side is well cooked, flip them and add a good squeeze of lemon juice.
Continue cooking until potatoes are roasted on the outside and tender in the middle. Reduce heat and cover if necessary. (If using a grill pan without a lid, you can cover it with a small baking sheet).
Small 1” potatoes should cook for a total of approx. 20-25 minutes.
Transfer warm cooked potatoes to the large bowl and toss with the dressing.
Let cool to room temp and toss with arugula, pickled onions, capers, chives and lemon zest.
Taste and add more dressing to your liking.
Serve at room temp, or make ahead and chill.
Notes
*If using an outdoor grill, you may need a pan or grill basket so the potatoes don’t fall through the grates. You could also boil your potatoes or roast them in the oven.