Preheat the oven to 425°F. Line a sheet pan with parchment paper.
In a medium bowl, combine ¼ cup olive oil and the harissa sauce. Add the sweet potatoes and toss well to coat. Scoop the potatoes from the marinade with a slotted spoon and place them on the prepared sheet pan in a single layer on one side of the pan. Place the onion wedges next to the potatoes and drizzle with a little olive oil and season with a good pinch of salt and pepper. Roast for 10 minutes.
Pat the chicken breast dry and season both sides with salt and pepper. Place the chicken into the bowl with the remaining marinade and toss to coat well. After the potatoes and onions have cooked for 10 minutes, add the chicken to the pan next to the onions. Roast for 20 minutes or until the chicken is cooked through and the potatoes are fork tender.