Bring 3 cups of water to a boil for blanching the mushrooms. While waiting, prepare the other ingredients.
Wash the shimeji mushrooms and cut off the root base. Gently separate them into individual stems by hand.
Slice the onion into thin half-moon pieces and cut the green onion diagonally into 1 cm pieces.
Make the Seasoning Sauce. In a small bowl, mix soy sauce, mirin, minced garlic, sugar, and sesame oil. Stir well and set aside.
Once the water is boiling, add the shimeji mushrooms and blanch them for 1 minute. Drain immediately using a sieve and shake off excess water.
Transfer the blanched mushrooms to a bowl and mix them with the prepared seasoning sauce. Toss well so they absorb the flavors. Set aside for a few minutes.
Heat a pan over medium heat and add vegetable oil. Add the sliced onion and stir-fry until it turns golden brown and caramelized.
Once the onion is browned, add the seasoned mushrooms and green onion. Stir-fry everything for about 2 minutes until well combined and heated through.
Transfer the stir-fried mushrooms to a plate and enjoy as a side dish with rice!