Slice beef steak against the grain into thin strips about 1/4 inch thick. Place in a bowl and toss with 1 tablespoon soy sauce, 1 teaspoon sesame oil, and 2 tablespoons cornstarch (from flour substitute). Let marinate for 15 minutes.

While beef marinates, prepare all vegetables: slice mushrooms, cut bell peppers into 1-inch pieces, mince garlic cloves, and grate fresh ginger. Have all sauce ingredients measured and ready.

In a small bowl, combine remaining soy sauce, oyster sauce, 1/4 cup water, and a pinch of black pepper to make the stir-fry sauce. Set aside.

Heat wok over highest heat until smoking. Add 2 tablespoons vegetable oil and swirl to coat. Add half the beef in a single layer and sear without moving for 1 minute. Flip and cook another 30 seconds. Remove to a plate. Repeat with remaining beef.

Add another tablespoon of oil to the wok. Add garlic and ginger, stirring constantly for 15 seconds until fragrant. Add mushrooms and stir-fry for 2 minutes until they begin to soften.

Add bell peppers and stir-fry for 1 minute. Return beef to the wok along with any accumulated juices.

Pour the sauce mixture over everything and toss vigorously for 30-45 seconds until the sauce thickens and coats all ingredients with a glossy sheen.

Remove from heat immediately and transfer to a serving platter. Garnish with sliced green onions if desired. Serve hot over steamed white rice.