Combine beef, salt and pepper. Marinate for about 10 minutes.
Combine Worcestershire sauce, soy sauce, brown sugar and red chili pepper flakes. Whisk until sugar is dissolved. Set aside.
In a cast-iron wok over medium heat, add butter and oil. When butter begins to melt, add garlic. Cook, stirring regularly, for about 1 minute or until garlic lightly browns.
Increase heat to high and stir garlic continously to prevent from burning. When the wok is very hot, add beef and spread across wok. Allow to sear for about 1 to 2 minutes and then turn to sear on other side.
Add Worcestershire-soy sauce mixture and continue to cook, stirring regularly, for about 3 to 5 minutes. Serve hot.
