Heat the oil in a large skillet over medium-high heat until hot. Add mushrooms, let sear, stirring frequently for 1 to 2 minutes.
Add garlic and stir until fragrant, about 30 seconds.
Sprinkle sugar and salt on top. Cook and stir until slightly charred, 2 minutes or so.
Pour soy sauce over mushrooms. Quickly stir for another minute, until the soy sauce is absorbed evenly and the mushrooms have softened. Add the green onion if using, stir a few times to mix well. Taste the mushrooms and adjust seasoning by adding more salt if needed. Turn off the heat and transfer the mushrooms to a plate. Serve hot as a side dish.
