Juicy beef strips are wok-tossed with colorful bell peppers and onions in a savory sauce. This high protein, veggie-loaded stir-fry supports a healthy calorie deficit and makes a vibrant family meal.
Slice the beef sirloin steak thinly against the grain and combine with 1 tbsp soy sauce and cornstarch; let marinate for 10 minutes.
Cut the red and green bell peppers and the yellow onion into thin strips. Mince the garlic.
Heat 1 tbsp vegetable oil in a large wok or skillet over high heat. Once shimmering, add the beef in a single layer and sear undisturbed for 1 minute, then stir-fry until just browned, about 2 more minutes. Remove beef from the pan.
Add the remaining 1 tbsp oil, then stir-fry the onions and peppers on high heat, tossing constantly, until crisp-tender, about 3-4 minutes.
Return the beef along with garlic to the pan. Add remaining soy sauce, oyster sauce, black pepper, and water. Stir-fry all contents together for 1 minute on high, ensuring everything is coated evenly. Serve immediately.