Thinly slice the onion and remove the root of the shimeji mushrooms, then separate them.

Pat dry the salmon fillets with a paper towel and cut them into bite-sized pieces. Season with salt and pepper.

Boil water in a pot and cook the macaroni according to package instructions. Drain and set aside.

Heat salad oil in a frying pan over medium heat. Place the salmon skin-side down and cook until both sides are browned and cooked through. Remove from the pan.

In the same pan, melt butter over medium heat. Add the onion and shimeji mushrooms, cooking until softened. Sprinkle flour evenly and stir until no longer powdery.

Gradually add milk, stirring continuously until thickened. Add the macaroni, cooked salmon, salt, and pepper, mixing well.