Step 1Heat the oil over high heat in a wide heavy pot large enough to hold the bones in 1 layer. When the oil just begins to smoke, add the bones. Sear the bones, without stirring until well browned, about 10 minutes. If the bones are moved before they are browned, they will give off their juices and begin to steam rather than brown. Turn the bones and cook until evenly colored, about 10 minutes more.
Step 2Add 1 cup water to the pot. Listen as the liquid goes into the pot: You will hear it sizzling as it hits the hot pot, then, as it reduces, it will become quiet. When the water has reduced to a syrup, about 15 minutes, stir with a wooden spoon, scraping up any glazed juices clinging to the bottom of the pot. Continue cooking until the water has completely evaporated and the pot is reglazed and sizzling again.
Step 3When the water has evaporated, deglaze the pot with 1/2 cup of chicken stock, following the same steps as the water. This time, as the stock boils down, the color of the bones and liquid will become deeper and the natural gelatin in the stock will glaze the bones.
Step 4Add the onions, leeks and carrots. The water in the vegetables provides enough liquid for a quick deglazing. Cook as above until the moisture has evaporated and the vegetables are lightly caramelized, about 10 minutes.
Step 5Add the remaining 2 cups chicken stock, the veal stock and the remaining 2 cups of water. Deglaze the pot, scraping up the glazed juices from the bottom, then transfer the stock and bones to a smaller, narrower pot so that it will be easier to skim.
Step 6Bring to a simmer with the pot set partially off the burner to force the impurities to the side of the pot and ladle them off as they rise to the top. Simmer until the stock has reduced to the level of the bones, about 1 hour.
Step 7Strain the sauce through a fine strainer and repeat. Do not force any of the solids through the strainer or they will cloud the sauce. You should have about 2 cups of liquid.
Step 8Pour the liquid into a small saucepan, reduce to about 1 cup and strain.