Cook unpeeled potatoes in boiling salted water until tender, about 25 minutes, and drain. When cool enough to handle, cut 8 of the potatoes in half. With a melon baller, scoop out the insides, leaving a shell about 1/8 thick. Peel remaining 2 potatoes and dice. Combine with onion, olives, mayo, mustard, 1/2 tsp. salt and 1/4 tsp. pepper. Sprinkle potato shells with salt and stuff with potato salad mixture.