Preheat your oven to 425°F for at least 10 minutes—this ensures the pan is screaming hot when chicken hits it, creating that essential caramelization. Line a large sheet pan with parchment paper or lightly oil it to prevent sticking and simplify cleanup.

In a small bowl, whisk together harissa paste, olive oil, lemon juice, minced garlic, cumin, paprika, coriander, cayenne pepper, salt, and pepper until smooth. This marinade paste should have a spreadable consistency resembling thick yogurt—adjust oil if it's too thick.

Pat chicken thighs completely dry with paper towels, then arrange them skin-side up on the prepared sheet pan. Rub the harissa mixture directly onto both sides of each thigh, ensuring even coverage because every exposed surface will caramelize during roasting.

Arrange bell peppers, cherry tomatoes, and red onion wedges around the chicken thighs in a single layer. The vegetables should nestle around (not under) the chicken so they roast properly and the chicken juices coat them. Drizzle any remaining harissa mixture over the vegetables.

Roast for 35 minutes until chicken thighs reach an internal temperature of 165°F at the thickest part. The skin should be deeply golden-brown and slightly charred at the edges, and the vegetables should be tender with caramelized edges.

Remove the stunning sheet pan harissa chicken from the oven and let it rest for 5 minutes before serving. This brief resting period allows the juices to redistribute throughout the meat, keeping every bite tender and flavorful rather than dry.

Scatter fresh cilantro over the entire sheet pan, squeezing additional lemon if desired for brightness. Serve family-style directly from the pan, letting guests help themselves and enjoying the theatrical presentation.