In a bowl, combine the sliced flank steak, minced lemongrass, garlic, ginger, soy sauce, fish sauce, brown sugar, and black pepper. Mix well and let marinate for at least 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the sliced bell pepper and snap peas, and stir-fry for 2-3 minutes until they are tender-crisp.
Return the beef to the skillet, add the chopped green onions, and stir everything together for another minute to heat through.
Serve the stir-fry over cooked rice or rice noodles.