Cook rice according to package instructions or in a rice cooker.
While the rice is cooking, heat a large skillet or wok to medium-high heat. Add cooking oil.
Add chopped onions and cook for a few minutes until translucent. Add garlic and cook for 30 seconds.
Add ground beef and cook until browned, stirring frequently.
Add lemongrass, fish sauce, brown sugar, and the cup of water. Cook stirring often until most of the water has evaporated, around 10 minutes.
Check the beef for seasoning level and add any salt and pepper if needed.