Preheat your oven to 425°F (220°C). Line a large sheet pan with parchment paper or a silicone mat for easy cleanup.
In a small bowl, whisk together harissa paste, 2 tablespoons of olive oil, minced garlic, cumin, paprika, lemon juice, salt, and pepper.
Pat the chicken thighs dry. Toss them in the harissa mixture until well coated.
Spread the cubed sweet potatoes, red onion wedges, and cherry tomatoes on the sheet pan. Drizzle with the remaining 2 tablespoons of olive oil and season lightly with salt.
Nestle the seasoned chicken thighs among the vegetables on the sheet pan. Try to give each piece some space for even browning.
Roast in the oven for 25–30 minutes, until the chicken reaches an internal temperature of 165°F (74°C) and the vegetables are tender and caramelized at the edges.
If you’d like a little more color, broil for an additional 2–3 minutes on high, watching closely to avoid burning.
Remove from the oven and let rest for 5 minutes. Serve with lemon wedges if desired.