Combine chicken and potatoes in a large bowl. Toss with 2½ teaspoons salt and ½ teaspoon black pepper. In a small bowl, whisk together harissa, cumin and 3 tablespoons oil. Pour over chicken and potatoes and toss to combine. Let stand at room temperature for 30 minutes, or up to 8 hours in the refrigerator.

Meanwhile, in a medium bowl, combine leeks, lemon zest, a pinch of salt and the remaining 1½ tablespoons oil.

Heat oven to 425 F. Arrange chicken and potatoes on a large rimmed baking sheet in a single layer. Roast 15 minutes. Toss potatoes lightly. Scatter leeks over pan. Roast until chicken and potatoes are cooked through and everything is golden and slightly crisped, 25-30 minutes.

While chicken cooks, place yogurt in a small bowl. Grate garlic over yogurt, and season mixture to taste with salt and pepper.

To serve, spoon yogurt over chicken and vegetables in the pan. Scatter arugula and dill over mixture. Drizzle with oil and lemon juice and serve.