Trim the spinach roots. Boil spinach for 1 minute, rinse under cold water, squeeze out excess water, and cut into 4cm pieces. Slice the onion and separate the shimeji mushrooms.

Cut chicken into bite-sized pieces and season with salt and pepper.

Heat butter in a cleaned pot over medium heat. Add chicken and cook until the color changes.

Add onion and sauté until softened. Add shimeji mushrooms and cook for 2 minutes.

Add flour and stir until no raw flour remains. Gradually add milk, stirring after each addition until thickened.