Cut the beef into thin strips across the grain. In a small bowl combine brown sugar, soy sauce, patis or fish sauce, lime juice, salt and pepper stirring well. Pour over beef and marinate for 10 to 15 minutes.
Heat oil in a wok or deep skillet over medium heat. Drain the beef, reserving the sauce, and fry the meat until tender for about 3 to 5 minutes then stir in the garlic until fragrant.
Add the beans, lime leaves or a little finely grated lime zest and the reserved sauce. Stir-fry for 2 to 4 minutes or until beans are tender. You may wish to add a little water if mixture is to thick and adjust seasonings according to your style and liking. Remove from heat, transfer to a serving platter. and garnish with coriander (cilantro) leaves. Serve immediately.