Pre-heat the oven to 180’C / 350’F or gas 4 and grease a shallow a shallow ovenproof gratin dish. Cut the leeks diagonally into 5 mm (1/4 in) slices and blanch in boiling water for 30 seconds. Drain, rinse under cold water, drain again and dry. Cook the onions in the butter for 2-3 minutes until softened. Pour the cream over the sliced potatoes in a pan. Add the garlic and season with salt, pepper and nutmeg. Bring to the boil then stir in the leeks and onions. Pour the mixture into the prepared dish to a depth of 4 cm (1 1/2 in), covering with all the cream. Place the dish into a roasting tray filled with very hot water and bake in the pre-heated oven for 1 hour until the potatoes are cooked through and golden brown on top. To check the potatoes during cooking, just pierce with a small vegetable knife. When it goes through easily you will know they are cooked. Remove the dish from the hot water. As a finish you can top with the grated Cheddar and glaze under a hot grill until golden brown.