Divide the flank steak evenly along the grain into two big pieces. Then slice each piece against the grain into 1/8” (3-mm) thick pieces. Transfer into a medium bowl and add the rest of the marinating ingredients. Mix well and let marinate while preparing the other ingredients.

Combine all the sauce ingredients in a medium-sized bowl. Stir to mix well.

Heat 1 tablespoon of oil in a large skillet over medium-high heat until hot. Spread the beef in the skillet in a single layer. Sear until the bottom turns golden without moving it, 1 minute or so. Stir and flip to cook the other side, until it turns golden, another minute or so. Transfer the cooked beef pieces to a big plate and set aside.

Pour in the remaining 1 tablespoon oil and add the butter, garlic and the scallion whites. Stir a few times to release fragrance.

Add the mushrooms. Cook and stir until the mushrooms are cooked through and the liquid has evaporated.

Stir the sauce again to thoroughly dissolve the cornstarch and pour it into the pan. Stir a few times until the sauce thickens.

Add the butter, then add the beef back into the skillet. Stir to coat everything well with the sauce. Add the scallion greens for garnish. Transfer everything to a big plate. Serve hot over steamed rice as a main dish.