Prep Beef: Add the beef, baking soda, cornstarch and 2 tbsp of soy sauce to a bowl. Toss everything together and set aside.
Combine Stir Fry Ingredients: Combine all the stir fry sauce ingredients together in a bowl and set aside.
Cook: Heat the olive oil in a large skillet or wok over high heat. Add the beef and spread it out. Don't touch it for 2 minutes, allowing it to sear. Toss it around a bit, then add the garlic, onion, ginger and cook for 1 minute. Stir in the mushrooms and cook for an additional 2 minutes, just until they start to brown a bit. Pour the sauce over everything and toss until well combined.
Garnish & Serve: Garnish with green onions and serve.
This will last 3–4 days in the fridge when stored in an airtight container! You can reheat it either in the microwave, or covered in a pan over medium heat.
If you want to keep leftovers longer, pop them into an airtight container and place in the freezer. Your leftover stir fry will keep for 3–4 months in the freezer.