Preheat the oven to 200C (400F / gas mark 6).
Toss the courgette, pepper and onion with half the olive oil, oregano, salt and pepper, and spread on a baking tray.
Roast for 20-25 minutes, turning halfway, until tender and lightly caramelised.
Meanwhile, rub the chicken with the remaining oil and season well.
Grill or pan-fry the chicken for 6-7 minutes per side until cooked through with no pink remaining and juices run clear.
Rest the chicken for 3 minutes, slice, and serve with the roasted vegetables.