Pull the skin off the raw thighs and reserve it. Pat the thighs dry and season with a pinch of salt. Heat the oil in a large pot over medium-high heat until shimmering, then sear the thighs until browned, 3 to 4 minutes per side. Remove the thighs to a plate, reduce the heat to medium, add the onion and garlic and cook until soft, about 3 minutes. Add the chicken broth and scrape up the browned bits from the bottom of the pot. Return the thighs to the pot, then add the fish sauce, bring to a boil, skim any foam, then simmer gently, partly covered, for 45 minutes. When the 45 minutes are up, lift out the thighs, shred the meat, discard the bones and set the meat aside.

While the broth simmers, heat the oven to 400°F. Lay the reserved skin flat on a parchment-lined sheet pan and bake until golden and crisp, 20 to 25 minutes.

Heat the oil in a small skillet over medium-low heat. Add the sliced garlic and cook, stirring often, until pale golden, about 5 minutes. Drain on a paper towel and set aside.

In a bowl, combine the pork, shrimp, egg, water chestnuts, chives, sesame oil, soy sauce, oyster sauce and salt, and mix until sticky. Pan-fry a teaspoon to taste the seasoning and adjust if needed.

Place 1 ½ teaspoons of filling in the middle of a wrapper, wet the edges, then gather the four corners and scrunch them together at the top to seal. Keep finished wontons under a damp towel. Repeat with the rest.

Bring the broth to a gentle boil. Add about 12 wontons at a time and cook until they float, about 4 minutes. Lift them into serving bowls with a slotted spoon. When all the wontons are cooked, return the shredded chicken to the pot. Taste and add more fish sauce or salt if needed. Turn off the heat as soon as it's hot.

Ladle the hot broth and chicken over the wontons. Top with fried garlic, crispy chicken skin (crumbled, or in large pieces for presentation), chives and chili oil.