Cut off the roots of bok choy and wash away any dirt and sand from the leaves.
Bring a large pot of water, add 1 tablespoon vegetable oil and 1 teaspoon salt to a boil. Blanch the Shanghai bok choy until they are just wilted but still bright green. Remove the bok choy and cool them down in a bowl of cold water for about30 seconds. Use a colander to drain the bok choy.
Arrange the bok choy into a circle around a large serving plate. Lay the leaves tightly next to each other.
Add 1½ tablespoons vegetable oil to the pan and heat over medium-high level. Add the mushrooms and stir fry for 30 seconds.
Add dark soy sauce, regular soy sauce, oyster sauce (or vegetarian oyster sauce), sugar,sesame oil, and white pepper. Stir until combined for another 10 seconds.
Mix the cornstarch and water. Slowly pour the slurry into the sauce while stirring constantly.
Re-season with more soy sauce, sugar, or oyster sauce according to personal preference.
Adjust the thickness of the sauce by adding more hot water or more cornstarch slurry. When you lift the spoon out of the sauce, it should be thick enough to leave residue on your spoon.
Transfer the mushrooms and sauce to the centre of Shanghai baby bok choy circle and serve this immediately for best taste.