Pat steak dry with a paper towel. Generously sprinkle it with salt and pepper.
In a hot pan, add neutral oil and render the fat cap. Once rendered, flip the steak on one side and sear until it forms a crust, about 5 minutes. Gently press down on all parts of the steak to ensure an even sear.
Flip the steak, add the smashed garlic, lemongrass stalk, and butter. Begin basting. Baste the steak until it reaches about 5-10 degrees before your desired temperature. For a medium rare, pull the steak when the internal temperature is about 120-125F. Pull it off the pan and let it rest for 10-15 minutes. Note: if you are making more than one steak and do not have a large pan to fit all the steaks, work in batches so as to not steam the steaks.
For the sauce, add minced cilantro, minced garlic, minced thai chilies, and sugar to a mortar and pestle. Gently smash them together. Add lime juice and Red Boat Fish Sauce. Mix to combine and adjust seasoning and spice as desired.
Once the steak has rested, slice against the grain and on a bias. Serve the steak with the Jeow Som dipping sauce.