Marinate the beef with soy sauce, dark soy sauce, cornstarch, and baking soda for 10 to 15 minutes.
Meanwhile, prepare the sauce by mixing soy sauce, oyster sauce, sugar, cornstarch, and water in a small bowl. Set aside.
Heat 1 tablespoon of oil in a wok or skillet over medium-high heat. Add the sliced garlic and fry for 1 minute on both sides, or until crispy and golden brown. Transfer to a plate lined with paper towels and set aside.
Turn the heat up to high, add the beef, and use the remaining oil to stir-fry until the beef is about 70% cooked. Remove from the pan and set aside.
Heat the wok or skillet with 1 tablespoon of oil over medium-high heat. Add the mushrooms and stir-fry until soft, about 1 to 2 minutes. Then, transfer the beef to the wok or skillet and stir-fry until cooked through and no longer pink in the center, about 1 to 2 minutes. Next, stir in the prepared brown sauce.
Once the sauce thickens, turn off the heat. Dish out and top with the crispy garlic slices. Serve immediately.