In a bowl, toss sliced beef with cornstarch, soy sauce, and a pinch of baking soda. Let sit for 10 minutes to tenderize.
Heat oil in a large skillet or wok over medium-high heat. Add marinated beef and stir-fry until browned and cooked through, about 3-4 minutes. Remove beef and set aside.
In the same pan, reduce heat to medium and sauté minced garlic until fragrant, about 30 seconds.
Add chicken broth, oyster sauce, rice vinegar, brown sugar, sesame oil, and red pepper flakes. Bring to a simmer.
If sauce needs thickening, mix 1 teaspoon cornstarch with 2 teaspoons water and add to pan, stirring until thickened.
Return cooked beef to the pan and add sliced green onions. Toss everything together to coat and heat through, about 1-2 minutes.
Season with salt and pepper to taste. Serve immediately over steamed rice.