Season chicken all over with salt and pepper. Transfer to a large bowl and add harissa, lemon juice, and garlic. Rub into chicken with your hands to coat. Let sit at room temperature 30 minutes.
Preheat oven to 425°. On a baking sheet, toss potatoes and onion with oil, cumin seeds, and ground cumin; season with salt and pepper. Spread in an even layer, then nestle chicken in with veggies.
Bake until potatoes are golden and an instant-read thermometer inserted into thickest part of chicken registers 165°, 30 to 35 minutes.
Top with cilantro. Serve with lemon wedges alongside.
