Place cut tenderloin steak tips in a medium bowl. Add soy sauce, sugar, fish sauce, garlic, salt and pepper. Mix together until steak tips are covered in marinade. Cover with plastic wrap and let chill in the fridge for 20-25 minutes.
Meanwhile, cook rice. To make the sauce, combine soy sauce, sugar, fish sauce, rice wine vinegar, and salt and pepper. Whisk to combine and let sit to allow sugar to dissolve.
Add rice bran oil to a large skillet and set over medium-high heat. Once skillet is hot, spread beef in a single layer. Cook for about 2 minutes on each side. Remove beef from pan and discard expelled liquids in pan.
Return beef to pan and let sear for about one minute. Remove from heat. Add green onions and sliced red onions. Add prepared sauce. Toss to combine. Serve immediately over a bed of watercress. Serve with lime wedges and rice.