How to Make It
In a small bowl, cover the rinsed capers with warm water. Let stand at room temperature for 30 minutes. Drain.
Meanwhile, in a large saucepan, cover the potatoes with cold salted water and bring to a boil. Lower the heat and simmer until tender, about 30 minutes; drain. While the potatoes are still warm, peel and slice them 1/4-inch thick. Transfer the potatoes to a large bowl.
In a small bowl, combine the olive oil, lemon juice and capers and season with salt and pepper. Pour two-thirds of the dressing over the potatoes and toss.
Arrange the potatoes, onion and sablefish on a platter. Drizzle with the remaining dressing, garnish with the parsley and lemon wedges and serve.