Remove the steak from the packaging and pat both sides dry with paper towels. Season both sides with salt and pepper.
Heat a cast iron (or regular) skillet over medium high heat. Once hot, add one tablespoon of olive oil and the steak. Cook for 3-4 minutes per side, or until the steak reaches 140 ℉ for medium doneness. I also like to sear the edges.
Set the steak aside on a cutting board to rest for 10 minutes. While it rests, prepare the dressing and chop the vegetables.
In a medium bowl, whisk together the fish sauce, lime juice, sugar and chopped Thai red chili peppers until the sugar is dissolved.
After the steak has rested, slice it thinly against the grain into bite sized pieces. Add the steak to the dressing and toss to coat. As the steak sits in the dressing, the lime juice will gradually turn any pink areas brown. This is normal.
Add the cucumbers, tomatoes, red onion and cilantro to the bowl. Toss until everything is evenly coated with the dressing.
Arrange the lettuce on individual plates or in a large serving bowl. Using a slotted spoon, spoon the Thai beef salad over the lettuce. Drizzle additional dressing from the bowl over the top, if desired. Enjoy!