Dice the beef into 1¼ inch thick cubes. Marinate with ½ teaspoon salt, ¼ teaspoon sugar, plenty of black pepper, and 1 teaspoon oil. Set aside to come near to room temperature.
Thinly slice red onion. Cut scallions into 1½ inch long pieces. Mince the garlic.
Arrange the lettuce, micro greens, and tomato slices on a serving plate.
In a bowl, combine all ingredients to make the soy sauce mixture. Taste and adjust the sauce to your liking, set aside.
Place a wok or a heavy-bottomed pan over medium-high. When the pan is hot, add oil and then add the beef cubes. Sear for 30-45 seconds until browned and then flip to the other side, sear for another 30-45 seconds or until browned. Work in batches if needed.
Add onion slices to the pan, stir-fry the beef and onion for about 30 seconds. Then add minced garlic and the white parts of scallions. Stir and cook briefly until garlic is fragrant.
Add the soy sauce mixture, shake the wok or stir to coat the beef with the sauce. Then add the butter, constantly stir and cook until butter melts completely and everything is combined. This step should be done very quickly to avoid over-cooking the beef.
Add the green parts of scallions, quickly mix together and then transfer the beef to the serving plate on top of the greens and tomatoes.