Preheat oven to 200°C. Line an oven tray with baking paper.
Pat chicken dry with paper towels. Mix oregano, lemon zest and 2 Tbsp olive oil in a large bowl, add chicken and toss to coat. Leave to marinate at room temperature for 15-20 minutes.
In a roasting dish toss butternut, onion, capsicum and garlic with thyme or rosemary and a good drizzle of olive oil, and season. Roast for about 20 minutes or until slightly caramelised.
Stir tomatoes, sugar and vinegar together in a pot and simmer for 5-10 minutes until slightly reduced. Season to taste with salt, pepper and a good glug of olive oil.
Season marinated chicken with salt. Heat a drizzle of olive oil in a large frying pan on medium heat. Brown chicken in batches on both sides; you don’t have to cook it right through.
Arrange chicken, roast vegetables and olives (if using) in a large baking dish. Spoon over tomato sauce and scatter over mozzarella. Bake for 8-10 minutes, then switch to grill for 2-3 minutes or until cheese is bubbly and golden. Scatter over basil leaves and serve.