Wash the mushrooms in cold water and put them on a towel to absorb the liquid.
Finely mince the garlic and green chilis. Transfer to a bowl and set them aside.
Finely chop the green onions.
Pour the oil into an iron cast skillet and heat it until it shimmers.
Add the oil to the bowl containing the minced chilis and garlic. Add in the sugar, soy sauce, and oyster sauce. Whisk until completely combined.
Return the iron cast skillet to the stove over medium heat.
Add the mushrooms and the chopped green onions, reserving about 1 tsp. for garnish.
Pour the sauce onto the mushroom mixture.
Bring to a simmer, cover, and cook for 2-3 more minutes.
Turn off the heat and sprinkle with the reserved green onions.