Roast the bones in batches in the oven at 250 °C top heat until deep brown.
Fry the onions in clarified butter in a very large pot. Once translucent, add the remaining vegetables bit by bit and roast along. A brown layer may form on the bottom of the pot.
Add the tomato purée and roast briefly until it darkens. Add the roasted bones and deglaze with the red wine. Top up with water if needed.
Bring to the boil and simmer gently for 3 to 5 hours. Set the lid on at an angle so steam can escape.
Pour the contents through a sieve in batches, collecting the liquid in a second pot. Reduce this to about a fifth.
Only right at the end, once the sauce has reduced to a fifth, take the pot off the highest heat and stir in the ice-cold butter cubes until the sauce thickens and turns glossy. Do not salt: at this point the sauce is so concentrated that it needs none.
Fill into sterilised small jars or tins and leave to cool. Once cooled they can also be frozen.
