In a bowl, season beef with salt and pepper to taste. Marinate for about 10 minutes.
In a small bowl, combine Worcestershire sauce, soy sauce, brown sugar, and red chili pepper flakes. Whisk together until sugar is dissolved and set aside.
In a wide, thick-bottomed pan over medium heat, add butter and oil.
When butter is warm and begins to melt, but NOT hot, add garlic. Cook, stirring regularly, for about 1 minute or until garlic lightly browns.
Increase heat to high, stirring garlic continuously to prevent it from burning. Make sure the pan is hot.
Add beef and spread across the pan. Allow to sear for about 1 to 2 minutes and then turn to sear on the other side.
Add sauce mixture and continue to cook, stirring regularly, for about 3 to 5 minutes.
Remove from pan and garnish with chopped green onions and fried garlic bits, if desired.