Char the saved bones under the broiler until they’re caramelized and the edges start to blacken in spots.
Mix up a bigger batch of the golden-ratio tare: 2 parts soy sauce, 2 parts sake, 2 parts mirin, 1 part white sugar. Total volume is up to you.
Drop the charred bones into the tare and simmer gently for 45 minutes to 1.5 hours. The cartilage releases gelatin, which gives the sauce body. The caramelized exterior of the bones infuses flavor into the liquid.
Reduce until the sauce coats the back of a spoon, then strain out the solids.