Trim the tough ends of asparagus with a peeler and cut into bite-sized pieces. Boil water in a pot, add a pinch of salt and asparagus, and cook for about 3 minutes. Drain the water.

Place clams and sake in a pot, cover with a lid, and heat over medium heat until the clams open. Remove the clam meat from the shells and strain the steaming liquid.

Return the strained steaming liquid to the pot, heat over medium, and add soy sauce and mustard. Mix well and bring to a boil, then turn off the heat.