In a large bowl, combine 2 tablespoons harissa, 3 tablespoons olive oil, 1/2 teaspoon cumin, 2 1/2 teaspoons of the salt, and 1/2 teaspoon black pepper to make the marinade.
Add the chicken thighs, chicken legs, and potatoes to the bowl and rub the marinade over them, working some under the chicken skin; let stand at room temperature for 30 minutes.
In a medium bowl, toss the sliced leeks with the lemon zest, the remaining 1/4 teaspoon salt, and the remaining 1 1/2 tablespoons olive oil.
Preheat the oven to 425°F (220°C) and position a rack in the center.
Transfer the chicken, potatoes, and any remaining marinade onto a large rimmed baking sheet in a single layer with the chicken skin side up; bake for 20 minutes.
Remove the sheet from the oven, toss the potatoes to expose other sides for browning, then scatter the leeks evenly over the chicken and potatoes.
Return to the oven and bake until the chicken is cooked through and the potatoes are tender and browned, about another 20 to 25 minutes.
While the chicken finishes, stir together the yogurt, minced garlic, the remaining salt and pepper, and a squeeze of lemon or lime juice in a small bowl to make the sauce.
Remove the baking sheet from the oven, dollop or spoon the yogurt sauce over the chicken and potatoes, sprinkle with the chopped fresh herbs, and serve.