Combine the onion and vinegar in a small bowl with a pinch of salt and set aside. Mix the paprika, cumin, chipotle flakes and 2 tbsp of olive oil in a large bowl, seasoning well. Add the courgettes and toss together until coated. Put a large frying pan on a medium-high heat and, once hot, add the courgettes – you will probably need to do this in two or three batches. Cook on each side for 2-3 minutes or until deep in colour and charred, then transfer to a plate lined with kitchen paper. Cover and set aside.