Kosher salt and freshly ground black pepper, to taste
1/2 oz. Thai rock sugar or 1 tbsp. granulated sugar
3 tbsp. plus 1 cup fish sauce, preferably Squid brand
2 1/2 lb. fresh wide rice noodles or 32 oz. dried noodles, cooked and drained
1 cup cilantro, roughly chopped
4 scallions, thinly sliced
1/2 large white onion, thinly shaved using a mandoline, rinsed under cold water, and drained
Sriracha sauce, for serving
1/2 cup fresh lime juice
1 jalapeño, stemmed and thinly sliced
Pat chicken dry using paper towels and set on a baking sheet fitted with a rack; season generously with salt inside and out. Chill, uncovered, overnight.
The next day, transfer the chicken to a large pot and add 1 gallon of water; boil. Reduce heat to medium; simmer until chicken is cooked through, about 40 minutes.
Using tongs, transfer chicken to a cutting board and let cool; shred meat, discarding skin. Return bones to broth; simmer, skimming as needed, until slightly reduced, 35–40 minutes.
Stir in sugar, 3 tbsp. fish sauce, and salt; strain broth into a clean pot. Add reserved shredded chicken; keep warm.
Divide noodles between bowls; top with broth and chicken. Garnish each bowl with some cilantro, scallions, onion, and sriracha. Stir remaining fish sauce, the lime juice, jalapeño, and black pepper in a bowl; serve alongside soup for dipping chicken.