2 oz. slab bacon, cut into 1⁄4-inch lardons
1⁄4 cup finely chopped cilantro stems and leaves
1 tbsp. prahok (or other Asian fish or shrimp paste)
6 kaffir lime leaves, thinly shredded
4 cloves garlic, roughly chopped
3 small shallots, roughly chopped
2 serrano chiles, stemmed, seeded, and finely chopped
2 stalks lemongrass, inner cores thinly sliced
1 1⁄2 inches galangal, peeled and thinly sliced
1 tbsp. long-grain white rice
2 (12-inch-long) taro stems or 4 oz. water spinach or Swiss chard stems, peeled, halved lengthwise and cut into 3-inch pieces
1⁄2 cup cooked white rice
1 tbsp. finely grated palm sugar or light brown sugar
1 tbsp. fish sauce
1⁄2 cup 1⁄2-inch cubed white sweet potato
2 tbsp. vegetable oil
4 cups chicken stock
1 cup unsweetened coconut milk
Thinly sliced long beans, mung bean sprouts, and thinly shredded basil leaves, for serving
Whole Vietnamese mint and sawtooth herb leaves, to garnish
In a small skillet, heat the bacon over medium and cook, stirring, until browned and crisp and its fat renders, about 6 minutes. Using a slotted spoon, transfer the bacon to paper towels to cool and discard the fat. Meanwhile, in a food processor, combine the cilantro with the fish paste, kaffir lime leaves, garlic, shallots, chiles, lemongrass, and galangal and purée into a lightly chunky curry paste.
Meanwhile, in a small skillet, heat the uncooked rice over medium and cook, tossing occasionally, until lightly toasted, about 14 minutes. Transfer the rice to a spice grinder, let cool, and then process until finely ground. In a pot of boiling salted water, cook the taro stems until tender, about 10 minutes. Using a slotted spoon, drain the stems and let cool. Squeeze the stems to drain all their water and transfer to a bowl along with the ground toasted rice, cooked rice, palm sugar, and fish sauce and stir until evenly combined. Add the sweet potato to the boiling water and cook until tender, about 5 minutes; drain.
In a large saucepan, heat the oil over medium. Scrape the curry paste into the pan and cook, stirring, until fragrant and beginning to brown, about 6 minutes. Stir in the chicken stock and coconut milk and bring to a boil. Reduce the heat to maintain a simmer and cook, stirring, until reduced slightly, about 10 minutes. Transfer soup to a blender and purée until smooth. Pour the soup through a fine sieve back into the saucepan and keep warm.
To serve, divide the rice mixture among 4 serving bowls, mounding it in the center. Ladle the soup around and then top with the bacon, sweet potatoes, long beans, bean sprouts, and basil, and, if using, garnish with the Vietnamese mint and sawtooth herb leaves.