In a medium sized bowl, toss sliced flank steak with cornstarch until fully coated, then set aside.
In another medium sized bowl, whisk together beef broth, vinegar, soy sauce, brown sugar, garlic, ginger, sriracha, and pepper until combined. Set aside.
Heat vegetable oil in a large skillet over medium-high heat.
Once hot, add in onion, bell peppers, and celery and cook, stirring regularly, until the veggies are tender, about 5-10 minutes.
Remove veggies and set aside, but reserve remaining oil in the pan.
To the hot oil, add in steak and cook for 2-5 minutes, or until nicely browned. Remove and set aside with the veggies.
To the hot pan, whisk in sauce, scrapping up any brown bits from the bottom.
Bring to a simmer, stir in the cooked veggies and steak, and simmer until the sauce has thickened, about 5 minutes.
Taste and re-season, if necessary, then serve with garnishes and rice and enjoy!
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