Start by marinating the sirloin steak. In a medium bowl, combine the sliced beef, 1 tablespoon of low-sodium soy sauce, and black pepper. Toss to coat the beef evenly and let it sit for about 10 minutes while you prepare the other ingredients.
In a separate bowl, whisk together the ¾ cup of low-sodium soy sauce, ¾ cup of water, maple syrup, rice vinegar, minced garlic, minced ginger, red pepper flakes, and cornstarch. This mixture will create a wonderfully sticky sauce that brings all the flavors together.
Heat 1 tablespoon of olive oil in a wok or large skillet over medium-high heat. Once hot, add the marinated beef in a single layer. Cook for about 3-4 minutes, stirring occasionally, until the beef is browned and cooked through. Remove the beef from the skillet and set it aside.
In the same skillet, add another tablespoon of olive oil if needed. Toss in the sliced red bell pepper and mixed vegetables. Stir-fry for about 3-5 minutes until they are just tender but still crisp. Add the drained water chestnuts and half of the chopped green onions to the pan, stirring to combine.
Return the cooked beef to the skillet with the vegetables. Pour the prepared sauce over the beef and vegetables. Stir well to coat everything evenly. Let it cook for an additional 2-3 minutes, allowing the sauce to thicken and become glossy.
Remove the skillet from heat and serve your Teriyaki Beef Stir Fry over prepared brown rice, white rice, quinoa, noodles, or cauliflower rice. Garnish with the remaining green onions and sprinkle with sesame seeds if desired.
