In a mixing bowl, whisk together the soy sauce, rice vinegar, sesame oil, sugar, minced garlic, grated ginger, and red pepper flakes.

Slice your beef into thin strips, about 1/4 inch thick.

Add the sliced beef to the marinade, ensuring all pieces are well-coated. Let it sit for at least 30 minutes.

Heat a large skillet or wok over medium-high heat.

Add the marinated beef in a single layer to the hot pan.

Cook the beef for about 3-5 minutes, stirring occasionally.

Optionally add vegetables like bell peppers or broccoli during the last minute of cooking.

Once cooked, remove the stir fry from heat and serve over rice or noodles.