Step 1

Preheat oven to 450°. Toss cauliflower with oil on a rimmed baking sheet. Roast, tossing occasionally, until tender and well browned, 35–45 minutes. Let cool slightly; chop into pea-size pieces. Transfer to a large bowl. Add chiles, lemongrass, lime leaves, fish sauce, and lime juice; toss well.

Step 2

Meanwhile, place rice in a medium skillet and set over medium heat. Toast, shaking pan constantly to keep rice moving, until evenly browned, 10–15 minutes. Transfer to a plate; let cool. Grind in spice mill to a semifine powder.

Step 3

Toss scallions, pea shoots, cilantro, mint, and 2 tsp. toasted rice powder into cauliflower mixture; season with salt. Serve with cucumber, lettuce, and remaining rice powder to make lettuce cups.