Sauté the onion in the olive oil 5 minutes, then add garlic and tomatoes, breaking up the tomatoes with a spoon. Bring to boil, reduce heat and simmer 10-15 minutes or until thickened. Meanwhile cook potatoes in salted water 7-10 minutes, until just tender. Drain well. Combine sauce, potatoes and half the olives in an ovenproof dish; season to taste with salt and pepper. Top with remaining olives and shredded Parmesan cheese and broil until golden.