Heat the oven to 200C/180C fan/gas 6. Cut the courgettes into quarters lengthways, then into batons. Tip them onto a baking tray and drizzle with the oil. Add the lemon zest and season with salt and pepper and a pinch of chilli flakes, if using. Toss until the courgette pieces are well coated. Roast for 15-20 mins, moving the courgettes around in the tray halfway through cooking, until softened and slightly charred.