To make the squab sauce, heat up a wide pan over high heat. Once it begins to smoke, add enough oil to coat the bottom of the pan, I'm using avocado oil. Turn the heat down to medium high and add in the bones in a single layer cooking for 10 minutes or until GBD golden brown and delicious. Give the bones a flip and, cooking them for an additional 10 minutes. A good sear is essential, the fond that develops is this is where the sauce gets its flavor.
For the first deglaze, add in 4 oz water using a flat wooden spoon to remove all the fond from the pan.
Once the pan is au sec, almost dry, and you hear the bones start to sizzle again, deglaze with 2 oz of chicken stock. As the stock reduces, the gelatin from the stock should start to coat the bones and should deepen in color.
Add the 3 oz carrots, 2.5 oz onions, 2 oz leeks and 2 oz of the mushroom trim. The water inside the vegetables will provide the liquid for deglazing, but if you notice that the fond doesn’t fully come up, add in some of the water. This will help prevent it from burning, which will add bitter flavor to your sauce. Cook until the vegetables are lightly caramelized and the pam is au sec.
Add the remaining 8 oz of chicken stock, 8 oz veal stock, and the remaining 8 oz of water. Scrape up any remaining fond from the bottom then bring to a simmer. Pull the pot half way off the burner to create a convection simmer, which will push all the impurities to the side. Using a spoon, scoop all the impurities off and into a cup with water. Dipping the spoon into the water will help prevent incorporating the impurities back into the stock.
Simmer for 30 to 45 minutes, skimming often, until the stock has reduced to the level of the bones. Strain the sauce into another pot and reduce by half or until nappe, to coat the back of a spoon. Add in thyme and let it infuse for 5 minutes.