Preheat grill. Wash red potatoes and pat dry. Slice potatoes into 1/4" thick disks. Toss them in 2 tablespoons olive oil and salt and pepper in a large mixing bowl.
Spray the grill grates with the non-stick cooking spray and lay potato slices down. Grill 5-8 minutes per side. Meanwhile, in a small mixing bowl whisk together mustard, vinegar, oil salt and pepper to make a vinaigrette. Stir in rosemary and olives.
When potatoes are tender, transfer them back to the large mixing bowl and pour vinaigrette over them and toss well while still warm. The potatoes will absorb the vinaigrette.