1) Preheat griddle over high heat.
2) Brush potatoes with oil, season with salt and pepper and grill until slightly charred.
3) Whisk the vinegar and mustard in a bowl, then slowly whisk in the olive oil. Whisk in salt and pepper, to taste.
4) Add the potatoes to the vinaigrette, then add the peppers, onions and the parsley, and toss to combine.