Cut the sweet potato into bite-sized pieces and soak in water to remove excess starch. Slice the onion thinly and separate the shimeji mushrooms into small clusters.

Season the chicken drumsticks with salt and pepper.

Heat butter in a pan over medium-low heat. Add chicken drumsticks and cook until browned. Push them to the side and sauté the onion until softened. Sprinkle flour evenly and stir until no raw flour remains.

Add water, salt, and shimeji mushrooms. Stir and bring to a boil. Cover and simmer on low heat for 7 minutes. Add sweet potatoes and cook covered for another 7–8 minutes until tender.