In a small bowl, combine 2/3 cup of the harissa and 2 tablespoons of the oil. In a large bowl, combine the chicken, sweet potatoes, and harissa mixture, tossing to evenly coat. Set aside at room temperature for 30 minutes.

Meanwhile, preheat the oven to 425°F. In a small bowl, combine the yogurt, remaining 1 tablespoon of harissa, and remaining 2 tablespoons of oil. Add salt to taste and set aside.

Transfer the chicken to a large rimmed baking sheet and bake for 10 minutes. (Leave the sweet potatoes in the bowl.)

Add the sweet potatoes to the baking sheet with the chicken and cook until the chicken is cooked through and the sweet potatoes are tender and browned, about 20 minutes, tossing halfway through.

Transfer the chicken and sweet potatoes to serving plates or a platter if you like. Drizzle the yogurt mixture on top, garnish with the lemon and herbs, and serve. (Alternately, serve the yogurt mixture at the table.)

Note: Harissa is a North African and Middle Eastern red chile paste or sauce. Look for it in the ethnic section of supermarkets and at Middle Eastern markets and specialty food stores. Its spiciness can vary from brand to brand—taste the one you buy and, if it seems particularly hot, consider using a little less the first time you make the recipe.