Cut the chicken into thin slices across the grain. Mix them with cornstarch, salt and water until well combined. Stir in oil to create a coating.
Mix the sauce
In a small bowl, mix light soy sauce, oyster sauce, dark soy sauce, cornstarch, black pepper, sugar, and water. Once the mixture becomes smooth, add sesame oil. Set aside.
Sear the chicken
Heat an empty wok over high heat until it smokes. Pour in 1 tablespoon of oil and swirl it around before putting in the chicken.
Spread out the chicken to sear it quickly. Flip and toss to cook further once the bottom side slightly browns. When the chicken is cooked (no more pinkness remains), transfer it to a plate for later use.
Stir-fry the mushrooms
Add the remaining ½ tablespoon of oil to the wok. Sizzle sliced garlic for 10 seconds or so, then add sliced mushrooms. Keep the heat high and toss constantly until they just begin to sweat.
Turn the heat down to low (or temporarily remove the wok from the burner if using an electric cooker). Return the chicken to the wok, then pour in sauce-well stirred beforehand.
Toss everything while the sauce thickens (this takes very little time). Finally, stir in chopped scallions then dish out to serve.