Preheat your oven to 220°C (200°C fan). Place a large sheet pan in the oven while it heats up — you’ll hear a slight sizzle when the hot tray hits the vegetables later, which helps them char beautifully.

Prepare the marinade: In a small bowl, whisk together the olive oil, lime juice, lime zest, smoked paprika, cumin, garlic powder, oregano, cayenne (if using), and a generous pinch of salt and pepper. The mixture should smell fragrant and look a deep, rusty red.

Toss the vegetables: In a large mixing bowl, combine the sliced bell peppers and red onions. Pour over half of the marinade and toss until every piece glistens and is evenly coated. The onions will start to look translucent and the peppers will take on a glossy sheen.

Spread the vegetables on the hot sheet pan: Carefully remove the heated tray from the oven — it will be very hot. Spread the vegetables in a single, even layer. You should hear a satisfying hiss as they hit the hot surface. Roast for 10 minutes, until the edges of the peppers begin to brown and the onions soften.

Add the prawns: While the vegetables roast, add the prawns to the remaining marinade and toss gently. After 10 minutes, remove the tray from the oven and push the vegetables to one side. Arrange the prawns in a single layer on the empty side. The prawns should look pink and glossy against the roasted vegetables.

Roast for another 5–7 minutes: Return the tray to the oven. Cook until the prawns turn opaque and curl into tight 'C' shapes, and the tails become bright pink. You’ll notice a wonderful aroma of charred lime and smoked paprika filling your kitchen. Do not overcook, or the prawns will become rubbery.

Rest and assemble: Remove the tray from the oven and let it rest for 2 minutes. This allows the juices to settle. Warm your tortillas in a dry pan or directly over a gas flame for about 30 seconds per side, until they blister and become pliable. Serve the fajita filling directly from the tray, garnished with fresh coriander, a dollop of sour cream, and slices of avocado.