Combine the chicken and the potatoes in a large bowl. Season with 2 1/2 teaspoons kosher salt and 1/2 teaspoon pepper, tossing as you season. Combine the harissa, cumin and 3 Tablespoons olive oil in a small bowl and stir together, then pour this over the chicken and potatoes, stirring to coat. Let this stand for 30 minutes or so.
Combine the leeks, lemon zest, 1/4 teaspoon salt and the 1 1/2 Tablespoons olive oil in a bowl and stir to coat.
Heat the oven to 425.
Put the chicken and potatoes on a sheet pan with a rim. Make sure they're not crowded or things won't crisp up. Bake for 20-25 minutes.
Use a spatula to stir the chicken and potatoes around a little. If there is a lot of fat from the chicken at this point, drain off some so your potatoes can get crispy, then add the sliced leeks. Continue baking until the chicken is done, another 25-30 minutes.
Stir together the yogurt, grated garlic clove and the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper.
When the chicken and vegetables are done, spoon the yogurt over the top, scatter the herbs over the top and drizzle a bit of olive oil over the top, then squeeze some fresh lemon juice over it all and serve.