Start by placing the diced beef tenderloin or flank steak in a bowl. Add the baking soda and 1 tablespoon of cornstarch, tossing to coat the beef evenly.
Add 1 tablespoon of soy sauce and 1 tablespoon of reduced-sodium soy sauce to the beef. Mix well and let it marinate for about 15 minutes while you prep the other ingredients.
Heat 1 tablespoon of olive oil in your wok or skillet over high heat. Add the sliced garlic and sauté for about 30 seconds until fragrant.
Increase the heat to high and add the marinated beef to the skillet. Stir-fry for about 3-4 minutes until the beef is browned and cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of olive oil. Toss in the quartered baby bella mushrooms and stir-fry for about 4-5 minutes until they are tender and golden brown.
In a small bowl, whisk together the oyster sauce, dark soy sauce, sugar, and 4 tablespoons of water. In another bowl, mix the remaining 1/2 tablespoon of cornstarch with a splash of water to create a slurry.
Return the cooked beef to the skillet with the mushrooms. Pour the sauce mixture over the beef and mushrooms, stirring to combine. Then add the cornstarch slurry and mix well, cooking for another 1-2 minutes until the sauce thickens.
Remove from heat and serve your Beef And Mushroom Stir-Fry immediately over steamed rice or noodles.