In a large bowl, add the chicken and sliced potatoes. Season with the harissa sauce, 3 tbsp olive oil, salt, pepper, cumin, and the juice of one lemon. Mix until the chicken and potatoes are evenly coated. (Try to get some of the harissa sauce under the skin of the chicken thighs). Cover the bowl and let the chicken and potatoes marinade at room temperature for 30 minutes.
Heat the oven to 400°F
Add the marinated chicken and potatoes, and onion to an aluminum foil or parchment-lined sheet pan. Arrange in an even layer. Drizzle on the remaining 2 tbsp of olive oil and a pinch of salt and pepper.
Roast uncovered for 40-45 minutes, or until an instant thermometer inserted into the thickest part of the chicken reads 165°F. Be sure to flip the potatoes halfway through (at 20 minutes).
During the last five minutes of roasting, add the olives to the dish.
Remove the sheet pan from the oven and add the feta cheese and herb garnish. Squeeze 1/2 lemon onto the entire dish.
Enjoy! Serve with yogurt, tzatziki sauce, or avocado crema!