1. Prep ingredients
Bring a medium saucepan of salted water to a boil. Halve and thinly slice ⅓ of the onion (save rest). remove kale leaves from stem and coarsely chop. Pick cilantro leaves and stems; finely chop stems, keeping leaves whole. Break 10 of the rice noodles into 2-inch pieces; keep remaining whole. Line a plate with paper towel.
2. Fry noodles
Heat 2 tablespoons oil in a small pot over medium-high until shimmering. Add noodle pieces and fry, stirring frequently, until noodles are puffed, doubled in size, and golden brown, 1–2 minutes. Use a slotted spoon to transfer fried noodles to prepared plate and sprinkle with salt. Set crispy noodles aside until ready to serve.
3. Start soup
To same pot, add half of the sliced onions; reduce heat to medium. Cook, stirring frequently, until golden brown, 2–4 minutes. Immediately add all of the red curry paste and cilantro stems. Cook, stirring constantly, until curry paste is very fragrant, about 1 minute.
4. Simmer soup
To pot with curry paste, add all of the chicken broth and coconut milk, fish sauce,, and 2 teaspoons sugar; whisk to combine. Add chicken, then bring soup to a boil. Simmer, covered, over medium-low heat, until chicken is cooked through, about 5 minutes. Season with ½ teaspoon salt.
5. Cook remaining noodles
While soup is simmers, add remaining uncooked noodles to boiling water. Cook, stirring to prevent sticking, until al dente, 6–8 minutes. Drain noodles and spoon into serving bowls.
6. Finish & serve
Add kale to soup; simmer, covered, until kale is crisp-tender, 1–2 minutes. Cut lime into wedges. Ladle soup into bowls with noodles and top with crispy noodles, remaining sliced onions, and whole cilantro leaves. Serve soup with lime wedges on the side for squeezing over. Enjoy!