When they are ready the courgettes will have sweated down but still retain some colour and shape. You’re looking for a certain mushiness without going so far as out-and-out pulp. Not that it matters: if you forget these are on the stove and let them cook until they reach the state of pure, undifferentiated sauce, you will still have something pretty heavenly on your hands. Besides, in Sicily, you will find that different cooks have different preferences: as ever, there is no one way to cook the same thing.