For the lamb short stock, you will need to cut up the bones, if the butcher has not already done this for you. Using a meat cleaver, cut the bones into approximately 2" -inch pieces.
Heat a heavy-bottomed pot to medium high heat and add the oil, followed by the lamb bones. Stir to coat the bones in the oil.
While the bones are browning, prepare your mirepoix. Roughly chop the onion, carrot, celery and leek. Squeeze the excess juice from the tomatoes and give them a rough chop. Set aside. Once the bones have browned, add the mirepoix, tomatoes and rosemary. Stir occasionally, and allow the vegetables to caramelize.
Next, deglaze with white wine and simmer until all of the liquid has evaporated. Add the chicken stock, bring to a quick boil and then turn the heat to low to let reduce slightly. Once reduced, add the demi-glace and continue to simmer for about an hour to an hour and a half on medium-low heat. If you like, you can jump ahead to Step 3 (Preparing the Garnish), while the stock reduces.
After the stock has reduced by about half and has a nice, dark golden color, strain the vegetables and bones from the stock. For an even smoother sauce, strain the stock once again through a fine mesh sieve into a clean pot. Bring the stock to a boil and skim off any excess fat. Turn the heat down to medium-low and let simmer for 20 to 30 minutes. Reduce the stock by about half or until it has a thicker, sauce-like consistency.
Lastly, check the sauce for seasoning. You may need to add a pinch of salt and pepper. Set the finished sauce aside while you prepare the garnish.