Add all ingredients to a gallon-sized zip-top plastic bag. Seal the bag and massage the mixture to coat the meat. Place the bag on a plate and refrigerate for at least 20 minutes or up to 4 hours.
STIR FRY:
In a small bowl, whisk to combine the beef broth, cornstarch slurry, and soy sauce. Set aside.
Heat a wok or large skillet over high heat. Once hot, add the oil. Add the steak (with juices), and spread it out in an even layer. Cook until the meat is browned, about 4 to 5 minutes, tossing occasionally.
Once the meat is browned, make a well in the center and add the onion, green bell pepper, and red bell pepper. Cook until the vegetables are just starting to soften, about 2 minutes, tossing occasionally.
Stir in the garlic, brown sugar, salt, black pepper, and beef broth mixture. Toss it around and cook until the sauce is thickened. Once the sauce is boiling, it takes about 30 seconds to 1 minute to thicken.
Remove from the heat. Stir in the green onion and sesame seeds, reserving some for garnish if desired. Serve with cooked rice if desired.
Notes
Nutritional information does not include rice.
If you are preparing this recipe as GLUTEN-FREE, just be sure to use brands of soy sauce and beef broth that are known to be GF.