To make the stock, heat a large pan over medium-high heat, adding enough rendered fat to coat the bottom of the pan.
Add the bones in a single layer, cooking until GBD golden brown and delicious, about 10 minutes.
Flip the bones and repeat, making sure the fond doesn't start to burn. The second side might take a little less time than the first side.
Deglaze with 250 ml of water, scraping up the fond as the water reduces. As the water starts to cook out, the gelatin from the bones will start to glaze the bones, and the bones will start to sizzle again, deepening in color, and start to build the fond again.
Add in 250 ml of chicken stock, scraping up all the fond. Cook until the pan is almost dry, then add the 125 grams of onions, 90 grams of leeks, and 135 grams of carrots, all cut into 1-inch pieces, and cook for 1 minute.
Deglaze with 500 ml of water, scraping up all the fond, then add 500 ml of both chicken stock and chicken jus. Bring the pan halfway off the burner to create a convection simmer, which will push all the impurities off to the side, then use a spoon to remove them. I like to use a cup with warm water to remove any impurities from the spoon. Cook this for 45 minutes at a low simmer, then strain and hold off to the side.