Heat 75g butter in a heavy-based pan until foaming and fry 1 finely chopped onion for about 5 mins. Cut the kernels off 3 corn cobs and add to the pan with 125ml water. Turn down the heat, cover and cook gently for 10 mins. Add 1 tsp sugar and 125ml single cream, season and cook, uncovered, for 5 mins more. Sprinkle the top with 50g grated cheddar or mozzarella and cook for a couple more mins until melted.