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.