Preparation
Dressing and rice powder
Step 1
Place rice in a medium skillet and set over medium heat. Cook, tossing often, until golden brown and deeply nutty–smelling, 12–15 minutes. Let cool, then finely grind in a spice mill or mortar and pestle; set aside.
Step 2
Meanwhile, stir lime juice, fish sauce, red pepper flakes, and sugar in a small bowl until sugar dissolves. Season with salt and black pepper.
Step 3
Do ahead: You can toast and grind the rice a few days in advance (as Suntaranon does). If the smokiness and aroma have faded at all, just toss it in briefly hot skillet to refresh.
Beef and salad
Step 4
Place beef in a large bowl and add fish sauce, sugar, and 2 Tbsp. oil. Let sit at room temperature 1 hour, or cover and chill up to 1 day ahead.
Step 5
Prepare a grill for medium-high heat or heat a large skillet over medium-high. Oil grate or lightly coat skillet with oil. Season beef with salt and black pepper and grill until browned underneath, about 2 minutes. Turn over and grill on other side just until lightly browned (meat should still be slightly pink in the center), about 45 seconds.
Step 6
Toss tomatoes, shallot, scallions, lemongrass, chiles, garlic, cilantro, mint, 1 lb. cucumbers, and 1 tsp. rice powder in a medium bowl to combine. Drizzle half of dressing over and toss to coat. Taste and season with salt and black pepper.
Step 7
Add beef to salad. Drizzle with more dressing as desired and top with more cucumber slices and 1 tsp. rice powder (save remaining rice powder in an airtight container at room temperature for another use). Serve with lime wedges for squeezing over.
