Cook rice using a 1:1.5 ratio of rice:water.
Meanwhile, cut beef into bite-sized cubes and place into a bowl.
Mince 3 cloves of garlic and place into bowl with beef along with granulated sugar, salt, and pepper. Drizzle lightly with 1 tsp of vegetable oil and fold to combine. Set aside.
Then, mince remaining 2 cloves of garlic and add to a small bowl. Add in brown sugar, soy sauce, fish sauce, and rice vinegar. Whisk to combine.
Pour mixture over beef and let marinate for 30 minutes up to an hour.
While the beef is marinating, slice cucumbers diagonally, thinly slice the tomatoes, and slice onions into batons. Then, rinse lettuce and pat to dry.
Heat a large wok over high heat, add 2 tbsp vegetable oil, and sauté garlic and onions until fragrant, around 1-2 minutes.
Add arrowroot powder into the bowl with the marinated beef and toss to coat evenly. Then, add to wok.
Cook beef until medium rare, approximately 3-5 minutes.
Serve the beef over a bed of lettuce, cucumbers, and tomatoes with a scoop of rice on the side.