In a large bowl or plastic bag, add in your skirt steak and marinate with soy sauce, sesame oil, lemongrass, and white pepper. Marinate in the fridge for 30 minutes, or overnight.
When ready to cook your steak, take it out at room temperature for 20 minutes ahead of time.
In a separate large bowl, toss together shallots, mint, cilantro, and tomatoes. Pour over your fish sauce, lime juice, water, sugar, chili flakes, and rice powder. Heat a cast iron skillet until sizzling hot, you can also do this on a grill. Grill and char your steak on all sides, about 2 to 3 minutes on each side, until cooked to medium-rare (my preference is at 130-135 degrees).
Transfer to a cutting board to rest for 5 to 10 minutes and thinly slice against the grain. Toss in with your salad and serve.
Notes
Toasted Rice Powder: Take about 1/4 cup of Jasmine Rice, or Thai Sticky Rice. Transfer your rice to a dry skillet for about 15 minutes on medium-high heat, tossing periodically to prevent burning. Transfer the toasted rice to a pestle and mortar or spice grinder, and start to grind the rice powder until it is a pretty thin powder. Save any leftovers for other Thai recipes, like our Thai Larb.