- Cut the steak of sirloin fat. Like at Panda Express, cut the steak into 1.25-inch pieces against the grain. 
- Mix the meat, sesame oil, and soy sauce in a medium-sized bowl. Mix thoroughly and whisk gently. The beef can be marinated in the fridge for up to a day, or for 15 to 30 minutes, covered in the bowl. 
- As you wait, cut the bell peppers, onions, green beans, and mushrooms. Stir-fry sauce should be combined in a small basin. 
- Heat up a sizable stir-frying pan or wok, preferably 12 inches in width, over medium-low heat. Add the meat and 1 tablespoon of oil. Turn the heat up to medium-high and start spreading them out into a single layer right away. Fry in the pan for two minutes without turning. 
- After the bottom side has browned, turn it over and cook the other side for two minutes using a spatula with a firm tip. To keep them warm, set them aside. 
- Add 1 tablespoon of oil to the same pan. Add ¼ tsp salt and sauté the onion, mushrooms, and green beans over medium-high heat until the mushrooms start to sweat and soften, about 1 to 2 minutes. After adding them, sauté the bell peppers for 30 seconds. 
- Put the steak back in the pan. Increase the heat to a high level. Before adding the sauce to the pan, give it one more stir. Mix all the ingredients for a minute. 
- Switch off the heat. Spoon onto a large serving platter. Warm up and serve with rice.