In a bowl, mix oysters with potato starch. Add water and gently wash the oysters, replacing the water until it runs clear. Devein the shrimp, wash, and pat dry.

Slice the onion thinly.

Boil water in a pot, add a pinch of salt, and cook macaroni slightly shorter than the package instructions.

Heat olive oil in a pan over medium heat. Add shrimp and oysters, sauté briefly, then add sake. Cover and steam on low heat for 3-4 minutes. Remove with juices and set aside.

In the same pan, melt butter and sauté onions over medium-low heat until soft. Add flour and stir until no raw flour remains. Gradually add milk, stirring constantly, until the mixture thickens.

Add macaroni, shrimp, and oysters to the sauce. Simmer for 2 minutes and season with salt.