Preheat the oven and prepare the sheet pan. Set your oven to 220°C (425°F) and allow it to fully preheat—this is crucial for crisp skin. Line a large rimmed baking sheet with parchment paper or foil for easy cleanup. The pan should feel very hot when you hold your hand near it.

Make the harissa marinade. In a medium mixing bowl, whisk together the harissa paste, olive oil, lemon juice, cumin, smoked paprika, coriander, minced garlic, salt, and pepper. The mixture will be a vibrant, brick-red colour and smell intensely aromatic with a sharp, spicy note.

Marinate the chicken. Pat the chicken thighs dry with paper towels—this step cannot be skipped, as moisture prevents browning. Add the thighs to the bowl with the marinade and toss well, using your hands to coat every surface, especially under the skin. You should see the marinade clinging to the skin in a thick, glossy layer. Let it sit at room temperature for 15 minutes while you prepare the vegetables.

Prepare the vegetables. In a separate bowl, toss the red onion wedges, courgette slices, and cherry tomatoes with a drizzle of olive oil and a pinch of salt. The onions should separate into petals, and the courgettes should be evenly coated. Spread the vegetables in a single layer on the prepared sheet pan, leaving space in the centre for the chicken.

Arrange the chicken. Place the marinated chicken thighs skin-side up in the centre of the pan, nestled among the vegetables. Make sure the thighs are not touching each other—overcrowding will steam the skin instead of crisping it. The pan should look vibrant, with deep red chicken and green and red vegetables.

Roast the chicken. Place the pan on the middle rack and roast for 30 minutes. After 30 minutes, check the colour: the skin should be deep golden-brown and starting to crisp. If it’s not, increase the oven temperature to 230°C (450°F) and roast for an additional 5-10 minutes. The internal temperature of the thickest part of a thigh should read 74°C (165°F) on an instant-read thermometer. The vegetables will be tender and caramelised, with the tomatoes beginning to burst.

Rest and garnish. Remove the pan from the oven and let the chicken rest for 5-10 minutes. During this time, the juices redistribute, and the skin stays crisp. You will hear a faint sizzle as the pan cools. Sprinkle generously with fresh coriander or parsley before serving.