Pour the oil into a baking tray and throw in the bones. Cook until nicely coloured. Stir on a regular basis in order to obtain a nice uniform colour. Do not let it burn as it will get bitter. Add the vegetables and let it cook a bit further. Now, place the bones into a big pot with the vegetables, deglaze the baking tray with the white wine while scrapping all the good stuff, pour into the pot, add in chicken stock (or water) and bring to the boil, add the bouquet garni and remove impurities. Add in tomato purée. Simmer for 8 hours.
Strain and let reduce if necessary.
If you want to make demi-glace, you need to let the veal stock reduce by one-third to half.