Chop garlic, ginger, chili, and green onions. Wash and dry lettuce leaves well.
Heat a skillet over high heat and add oil. Add ground beef and let it brown without stirring.
Cook until deeply browned and slightly crispy, then remove excess fat from the pan.
In the same pan, add garlic, ginger, and chili. Sauté until fragrant.
Return beef to the pan. Add fish sauce, soy sauce, and brown sugar. Stir well.
Turn off heat. Stir in lime juice, sesame oil, and green onions.
Spoon beef into lettuce cups and serve immediately.