Cut the beef - To make the flank steak more tender, always cut against the grain direction. (Note 1) You can refer to my other post, "How To Velvet Beef For Stir Fry?" I'll show you how to cut the beef and how to tenderize it. Marinate beef - Marinate with all ingredients. Mix well and marinate for at least 10 minutes (you can also leave it overnight). Add cornstarch 10 minutes later. (Note 2)
Pan-fry beef - Add the canola oil to a large frying pan and heat on medium-high heat. Add the steak to the pan in a single layer (to get a good sear) and cook on each side for 1 minute. If you overcrowd the pan, your steak won't be seared, it'll be steamy. When the steak is done searing, remove it from the pan. Beijing Sauce- Mix brown sugar, soy sauce, oyster sauce, ketchup, cornstarch, vinegar, sweet chili sauce, and water in a small bowl. Set aside.
Prepare the vegetables- Cut onion and red bell pepper into 1” chunks. Chopped fresh garlic. Set aside.
Fry the spices - In the same pan, add the remaining tablespoon of oil, and the garlic, and fry them for 10-15 seconds.
Add the sauce to the pan and simmer for 1 minute.
Add the onion, stir for 1 minute.
Return the beef and add red bell pepper- Add the beef and red bell pepper back in, if you want the beef crispy, you don't have to toss it in the Beijing sauce for too long, about 1 minute is enough.
Ready to serve.