Heat a large skillet over medium-high heat. Add the oil and then stir in the onions. Cook for about 2 minutes or until the onions are soft.
Add the garlic, mushrooms and green onions and stir occasionally until the mushrooms are lightly browned and tender, 3 to 5 minutes.
Stir in the soy sauce, sesame seed oil and crushed red pepper flakes. Cook until heated through.
Season with salt and pepper if desired.