In a large mixing bowl, combine the sliced napa cabbage, ground pork, minced shrimp, ginger, soy sauce, rice wine vinegar, sugar, and black pepper. Mix thoroughly with your hands or a spatula until the filling is evenly combined and slightly sticky.

Place about 1 tablespoon of filling in the centre of one wonton wrapper. Dip your finger in water and moisten all four edges. Place a second wrapper on top and press firmly from the centre outward to push out air, then seal the edges tightly. Repeat with remaining filling. To freeze, arrange wontons in a single layer on a parchment-lined baking sheet, freeze until solid (about 2 hours), then transfer to a zip-top bag for up to 2 weeks.

Place chicken bones, white parts of the green onions, ginger slices, soy sauce, peppercorns, and salt in a large stockpot. Add enough cold water to cover the bones by about 2 inches (roughly 12-14 cups / 3-3.5L). Bring to a boil over high heat, then reduce to a gentle simmer. Skim any foam or impurities that rise to the surface during the first 20 minutes. Simmer uncovered for 2½ hours. In the final 10 minutes, add the shiitake mushrooms and rice wine vinegar. Strain the broth through a fine-mesh strainer, discarding the solids. Taste and adjust salt.

Bring a large pot of water to a rolling boil (this is separate from the broth). Add half the wontons and stir gently to prevent sticking. When the wontons float to the surface (about 2-3 minutes), add 1 cup (240ml) of cold water to the pot. Let the wontons return to a boil and float again (about 1-2 more minutes). Remove with a slotted spoon and transfer directly into bowls. Repeat with the remaining wontons.

To serve, divide the cooked wontons among 4 bowls (about 8-12 per bowl depending on size). Ladle the hot strained broth over the wontons and garnish with thinly sliced green onion tops.