Prepare the Beef: Cut the beef into thin strips, about 1/4-inch thick, against the grain. In a bowl, marinate the beef with sesame oil, baking soda, cornstarch, and salt for about 15 minutes. Season with salt and pepper.
Cook the Beef: Heat a wok or large skillet over medium-high heat and add a small amount of oil. Cook the marinated beef in batches, browning each side for 1-2 minutes until it's no longer pink. Remove the cooked beef from the pan and set aside.
Prepare the Sauce: In a small bowl, whisk together the soy sauce, hoisin sauce, brown sugar, minced garlic, and ginger. In another bowl, mix cornstarch with a little water to create a slurry.
Cook the Vegetables: In the same pan, add more oil if needed and add your mixed vegetables. Stir-fry for 3-4 minutes, or until they're tender-crisp.
Combine Everything: Return the cooked beef to the pan with the vegetables. Pour the sauce over the mixture and stir constantly to combine. Add the cornstarch slurry to thicken the sauce, cooking until it reaches your desired consistency.