In a large frying pan, heat the oil and butter together over medium heat. Add the leeks and sauté for at least five minutes, so that they start to soften. Add the cream (to your desired thickness) and cheese, stirring thoroughly. Reduce the heat to low and cook gently for another 10 minutes until the mixture is thick and all of the cheese is melted.
Season with salt and pepper. Allow to cool slightly for a couple of minutes before serving.