Place leftover turkey bones in a large Dutch oven or stockpot. Fill with tap water and clean off the bones by either shaking the pot or using your hands to stir the bones to bring up loose bits. Drain the rinse water. Repeat, if necessary, until water runs pretty clear.
Fill the pot with 4-6 quarts cool clean water, or whatever is enough to cover the bones by at least 1-inch. Set the pot over medium-high heat to begin the cooking process while you prepare the aromatics.
Turn on another burner on your gas stove to medium-high. Using a pair of metal tongs (and wearing a heatproof mitt or glove), place the onions and ginger directly on the burner over the flame, turning them occasionally until they are charred black all over, about 10 minutes. If you don't have a gas stove, broil onions and ginger turning occasionally, for 15 to 20 minutes until charred on all sides.
Add the charred onions, charred ginger, smashed garlic, Korean pear or apple, cilantro stems, and spices to the pot.
Bring the pot back to a simmer, cover with a small vent, and cook for 1 hour and 30 minutes.
Remove large turkey bones to make straining easier. Pour the remaining contents of the pot through a mesh strainer into another large pot. Discard bones and ginger, onion, and pear solids. Stir in fish sauce, taste, and if needed, season with sea salt.