you will need a 25 litre pot for this
preheat the oven to 200ºC
roast marrowbones for 1 hour
transfer bones to 25 litre pot. Add 1 bottle of red wine and cover with water. Bring to an open, very low simmer (no sign of bubbling) for 2 days skimming scum from the top several times
strain through a tea towel and reduce until 3 cm liquid remains
red wine reduction:
in another large heavy based pan, place shallots, onions and garlic cut side down and cook until caremelised. Add the carrots, celery and 1 bottle of red wine. Reduce this for 1 hour to about 3 cm.
pour wine reduction together with vegetables into the large pot with the marrowbone reduction, boil and reduce to 3 cm and sieve into a clean pan.
add 2 tbsp good brandy, 1 tbsp tomato ketchup and seasoning, and bring to a slow boil
this should now cool down to a firm gelatinous sauce