Get your oven and tray ready. Preheat your oven to 200°C (fan-assisted). Line a large, rimmed baking sheet with baking parchment. This isn't just for easy cleanup — the parchment helps the peppers steam slightly before they caramelise, giving you that perfect tender-yet-charred texture.

Mix the spice rub. In a small bowl, combine the Cajun seasoning, smoked paprika, dried thyme, garlic powder, onion powder, and cayenne if using. Take a moment to inhale the mix — it should smell earthy, warm, and slightly sharp. That's your cue that it's balanced.

Prepare the chicken. Pat the chicken breasts dry with kitchen paper. This is crucial: any moisture on the surface will steam the spices rather than letting them toast. Drizzle 1 tablespoon of olive oil over the chicken and rub it all over. Sprinkle over half of the spice mixture, turning the breasts to coat them evenly. Set aside.

Toss the vegetables. In a large bowl, combine the sliced bell peppers, red onion wedges, and sliced garlic. Drizzle with the remaining 2 tablespoons of olive oil and scatter over the rest of the spice mixture. Use your hands to toss everything together — you'll feel the vegetables becoming slick and fragrant. Spread them out in a single layer on the prepared baking sheet. You should hear a gentle sizzle as the oil hits the pan if your tray is hot.

Nestle the chicken in. Place the seasoned chicken breasts among the peppers, skin-side up. Don't bury them — they need direct heat to brown. The peppers should be touching the chicken slightly, so their juices mingle as they roast.

Roast with confidence. Place the tray in the preheated oven and roast for 25 minutes. After this time, the peppers will be softening and the chicken will be just cooked through. Look for the chicken to be golden-brown on top, and the peppers to be collapsing at the edges with little charred spots. The smell at this stage is intoxicating — smoky, sweet, and slightly piquant.

Finish under the grill. For that final touch, switch the oven to a high grill setting. Grill the tray for 3–5 minutes, watching closely. You want the chicken to develop a few dark, crispy patches and the peppers to char slightly at the tips. The sizzling sound will intensify. Remove when the chicken reaches an internal temperature of 74°C.

Rest and serve. Let the tray rest for 5 minutes. Squeeze the lemon juice over everything — the acidity cuts through the richness and wakes up all the spices. Scatter with fresh parsley and serve directly from the tray for a wonderfully rustic feel.