Start by placing 450 g clams in a colander and rinse with running water. Prepare a bowl and add 500 ml water and 2 tsp salt. Mix well and then add the clams. Leave to soak for 1 hour to purge them of sand.
Drain the water and rinse the clams once more to remove any leftover sand or debris. Transfer to a large pot and add 3 tbsp sake.
Heat on medium and once the sake starts to bubble, reduce the heat to simmer and cover with a lid. Steam with the lid on for 5 minutes or until all of the clams have opened.
Remove the lid and add ½ tbsp butter and 1 tsp Japanese light soy sauce (usukuchi shoyu). Gently shake the pan to mix the butter and soy sauce until melted.
Serve up, drizzle with the sauce leftover in the pan and sprinkle with finely chopped green onions. Enjoy!