Beef tenderloin dice into 1 in. cubes. Marinate beef with 1 tbsp light soy, 1 tbsp fish sauce, quarter tsp dark soy, 2 tbsp sugar, 2 tsp black pepper and 2 tsp of oil.
Cover and marinate for 1 hour or 2.
Slice half onion. Dice 4 cloves of garlic.
Heat cast iron pan until smoke. Drizzle veg. oil and brown beef 1 min. per side. Put butter in, add all onion slices and garlic. Continue cook for 1 min.
Ta-Da