How to Make It
Remove the stem and seeds from the ancho chiles and pat dry. In a small skillet, heat the vegetable oil until shimmering. Add the anchos and fry over moderately high heat, turning once, until blistered and fragrant, about 2 minutes. Blot the anchos with paper towels and transfer them to a food processor. Let cool.
Add the serrano chiles, fish sauce, lime zest and juice and sugar to the food processor and puree until smooth. With the machine on, drizzle in the olive oil. Season with salt and transfer to a large bowl.
Add the tuna, onion, tomatoes, celery, hearts of palm and capers to the chile dressing and toss well to coat. Season with salt. Cover and refrigerate for at least 20 minutes.
Transfer the salad to a serving dish. Sprinkle with the rice powder and garnish with mint and cilantro leaves. Serve chilled, with toasts or sticky rice.
Roasted rice powder, known as khao kua pon in Thailand, is popular in East Asian cooking for its toasty flavor and its binding effect. It’s available at Asian markets. Or, you can make it: In a small skillet, toast raw glutinous (sticky) rice over moderately high heat, stirring occasionally, until golden brown and fragrant, 3 to 4 minutes. Transfer the rice to a mortar or spice grinder and let cool completely. Grind the rice to a coarse powder.