In a large skillet, heat the vegetable oil over medium-high heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and ginger to the skillet, cooking for an additional 1-2 minutes until fragrant.
Stir in the sliced flank steak and cook until browned, about 5-7 minutes.
In a small bowl, whisk together the soy sauce, brown sugar, fish sauce, rice vinegar, black pepper, and red pepper flakes (if using). Pour this mixture over the beef in the skillet.
Reduce the heat to medium-low and simmer for 10-15 minutes, stirring occasionally, until the sauce thickens and the beef is tender.
Remove from heat and let it cool for a few minutes. Serve over cooked jasmine rice and garnish with fresh cilantro.