Preheat the Oven: Set your oven to 425°F and allow it to heat up fully before roasting.
Season the Chicken and Potatoes: In a large mixing bowl, combine the chicken thighs and drumsticks with the potato chunks. Sprinkle 2 ½ teaspoons of kosher salt and ½ teaspoon of black pepper evenly over them. In a separate small bowl, mix together the harissa, cumin, and 3 tablespoons of olive oil until smooth. Pour this mixture over the chicken and potatoes, tossing everything to coat thoroughly. Let the mixture marinate at room temperature for 30 minutes.
Prepare the Leek Mixture: While the chicken marinates, place the sliced leeks in a medium bowl. Add the grated lemon zest, ¼ teaspoon of salt, and 1 ½ tablespoons of olive oil. Stir well to ensure the leeks are evenly coated, then set aside.
Start Roasting the Chicken and Potatoes: Spread the chicken pieces and potatoes in a single layer on a large rimmed baking sheet. Place the sheet in the oven and roast for about 20 minutes. Remove the baking sheet, toss the potatoes lightly to promote even cooking, and scatter the prepared leeks over the chicken and potatoes.
Finish Roasting: Return the baking sheet to the oven and continue roasting for another 20–25 minutes. The chicken should be fully cooked through, with golden and crisp skin, and the potatoes should be tender and browned.
Prepare the Yogurt Garnish: While the chicken and vegetables are finishing in the oven, grate the garlic clove into a small bowl of yogurt. Stir well and season with ¼ teaspoon each of salt and pepper. Thin the yogurt with a small splash of milk if needed to achieve a drizzling consistency.
Assemble and Serve: Once everything is roasted, drizzle the garlic yogurt over the chicken and vegetables directly on the baking sheet or transfer them to a serving platter first. Garnish generously with fresh herbs like dill, parsley, mint, or cilantro. For added flavor, drizzle a bit of olive oil and a squeeze of fresh lemon juice over the top. Serve warm.