Place the butter and oil in a large frying pan and gently heat. Cut the courgettes at an angle into slices about 2.5cm thick. Increase the heat under the oil and melted butter and add a layer of the sliced courgettes, frying them on each side until golden. Remove and keep warm, and repeat until all the courgettes are cooked. Return the courgettes to the pan, add the lemon zest and juice and the seasoning, toss and serve at once.