Mix all the sauce ingredients in a small bowl and set aside.
Add 1/4 cup water to a large skillet and heat over medium high heat. When the water starts to boil, spread the Chinese broccoli in the skillet, and sprinkle with pinch of salt. Cover and let steam until the Chinese broccoli just turns tender, 2 to 3 minutes, depending on the size. Flip once or twice in between and check the doneness. Transfer to a big plate when cooked.
Wipe the residual water with a few layers of kitchen paper towel held in a pair of tongs. Add the oil and garlic. Stir a few times to release the fragrance.
Add the mushrooms. Stir and cook until golden, about 2 minutes. Turn to medium low heat.
Stir the sauce again to fully dissolve the cornstarch. Pour sauce into skillet. Immediately stir until it thickens. Pour the sauce over the cooked Chinese broccoli.
Serve hot as a side.
Other types of mushrooms work, too.
The dark soy sauce adds an appetizing dark brown color to the sauce. You can skip it if you do not have any on hand.