Preheat the Grill: Set your outdoor grill to medium-high heat, around 400°F to 450°F.
Marinate the Chicken: In a large mixing bowl, add the chicken, 2 tablespoons of harissa, 1 tablespoon of olive oil, and 2 teaspoons of salt. Use a Microplane or fine grater to zest the lemon directly into the bowl. Cut the lemon in half, squeeze the juice from one half over the chicken, and set aside the other half for later. Toss the chicken until it is evenly coated with the seasoning.
Prepare the Zucchini: Slice the zucchini in half crosswise, then cut each half into ¼-inch thick planks. Place the zucchini on top of the marinated chicken in the bowl. Drizzle the remaining tablespoon of olive oil over it and season with ¾ teaspoon of salt. Gently toss everything together, ensuring the zucchini is coated but not overly mixed with the chicken.
Grill Everything: Arrange the chicken and zucchini on the grill. Cook each side for about 5 to 7 minutes, flipping once. The chicken should reach an internal temperature of 165°F, and the zucchini should become very tender. Once done, remove everything from the grill and transfer to a serving platter. Let the chicken rest for a few minutes.
Prepare the Sauce: In a small bowl, mix the yogurt (or labneh/sour cream) with the remaining teaspoon of harissa, the juice from the reserved lemon half, and ¼ teaspoon of salt. Stir well, then taste and adjust with more harissa or lemon juice if needed.
Serve: Tear fresh herbs over the top to add a pop of color and flavor. Serve the grilled chicken, zucchini, and yogurt sauce together and let everyone enjoy!