How to Make It
Light a grill or preheat a grill pan. Season the steak with salt and pepper. Grill over high heat until medium-rare, about 4 minutes per side. Let rest for 30 minutes.
Meanwhile, in a small bowl, whisk the fish sauce with the lime juice, sugar, lemongrass, chile, garlic and white pepper. In a large bowl, toss the tomato with the shallot, cucumber, mint, cilantro, basil and roasted rice powder.
In a large saucepan, heat 2 inches of oil to 375°. Line a baking sheet with paper towels. Fry the potato slices in batches until golden and crisp, about 4 minutes. Transfer to the lined baking sheet. Immediately sprinkle with salt.
Slice the steak against the grain 1/4 inch thick. Add the steak to the salad along with the dressing and toss. Add the potato crisps and toss gently. Spoon the salad into the lettuce leaves and serve.
Roasted rice powder, known as khao kua pon in Thailand, is available at Asian markets, but you can also make it at home: In a skillet, toast raw white sticky (glutinous) rice over moderately low heat, stirring occasionally, until lightly browned, about 10 minutes. Transfer to a mortar or spice grinder and let cool completely, then grind the rice to a powder. The rice powder can be stored in an airtight container for up to 3 months.