Season beef with salt pepper and a little fish sauce. Heat up cast iron or large skillet then add in a neutral oil. Sear the beef for 3 minutes without touching it. Then flip and cook for an additional 3 minutes. Once you get the desired doneness, reduce the heat then add in unsalted butter and garlic clove. Baste the steak for 1 minute. Remove from the pan and allow the steak to rest before slicing.
In a mortar and pestle mash down garlic cloves and Thai chilies to create a paste. Next, pour in fish sauce, vinegar, lime juice and sugar. Mix until the sugar is dissolved. Add warm water if needed. Set aside.
In a large bowl, combine lettuce, cucumber, shallots, tomatoes, mint, cilantro and green onions. Add in the sliced beef. And top with the dressing. Toss everything together and enjoy!
Notes
Make sure beef is at room temperature to get an even cook. Heat up cast iron until to just before it smokes. Then add a neutral oil. Sear the beef for 3 minutes without touching it then flip. Cook for an additional 3 minutes. Reduce the heat, then add in unsalted butter and a garlic clove and base the steak for an additional 1-2 minutes. Allow the steak to rest for 10 minutes before slicing. When slicing the beef, make sure to cut against the grain.