Step 1

Dressing and rice powder: 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.

Step 4

Beef and salad: 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.