In a bowl, mix together the oyster sauce, soy sauce and brown sugar until the sugar has dissolved. Mix the cornflour and water in a small bowl, and stir it into the oyster sauce mixture until smooth.
Fill a pot with water, bring to a boil, and stir in salt and vegetable oil. Place the bok choy into the boiling water and cook for just 2 to 3 minutes. Drain the bok choy, and arrange on a serving platter.
Heat the olive oil in a large wok then toss the spring onion and garlic in the hot oil until fragrant, about 20 seconds; stir in the mushrooms. Cook and stir until the mushrooms begin to shrink slightly. Sprinkle the mushrooms lightly with salt. Continue to cook the mushrooms, stirring often, until they are tender, about 5 minutes.
Pour in the oyster sauce mixture, then stir until the sauce is thickened and coats the mushrooms, 30 seconds to 1 minute. Pour the thickened mushroom sauce over the bok choy; serve immediately.