Preparation Cut potatoes into 1 1/2-inch pieces and cover with cold water in a large pot. Add 1 1/2 teaspoons salt and simmer over medium heat, partially covered, until just tender, 12 to 15 minutes. While potatoes cook, whisk together vinegar, mustard, sugar, and 1/4 teaspoon salt in a large bowl until sugar and salt are dissolved. Add oil in a slow stream, whisking until combined. Drain potatoes well and, while still hot, stir into dressing to coat, then stir in mackerel, scallions, and dill. Serve warm or at room temperature. Cooks' note:Smoked mackerel is available at specialty foods shops and many supermarkets, and by mail order from Ducktrap River of Maine (800-828-3825; ducktrap.com).