Peel the potatoes and cut into 3/4-inch dice. Place the baking potatoes in a large saucepan with 2 quarts cold, lightly salted water. Bring to a boil and cook for 4 minutes. Add the sweet potatoes and simmer until both types of potatoes are just tender, 4 to 6 minutes, 10 to 12 minutes in all. Drain the potatoes in a colander.
Meanwhile, prepare the dressing. In a large salad bowl combine the mayonnaise and mustard and whisk until smooth. Whisk in the oil, vinegar, capers, green olives, and pepper. Stir in the drained potatoes while still hot and let cool.
Stir in the celery, eggs, scallions, onion, and parsley. Correct the seasoning, adding salt or vinegar: the salad should be highly seasoned. Clean the edges of the bowl with a rubber spatula or a paper towel. Decorate the top of the salad with a black olive and parsley sprigs and serve at once.
Recipe courtesy of Steven Raichlen, author of The Barbecue Bible and Barbecue Bible Sauces, Rubs, and Marinades (Workman Publishing)