Preheat the oven to 200C/400F/Gas 6. If your butcher hasn't done so already, chop the bones into manageable pieces with a cleaver. Place the bones in a large roasting tin.
Cut the onions in half and add them to the tray with the bones. Cut the garlic in half across the middle and add to the pan. Pour half an inch of water into the tin and roast the bones and vegetables in the oven for 40-45 minutes, topping up the water occasionally, if necessary.
Meanwhile, place a large stockpot on the biggest hob ring. Fill with 4-5 litres/7-9 pints of cold water (about two-thirds full) and bring the water up to simmering point.
When the water is simmering and the bones are roasted, remove the bones, onions and garlic from the oven and add them to the water, scraping in the browned bits from the tin.
Add the bouquet garni, leeks, carrots, celery, tomatoes, salt and peppercorns.
Gently simmer the stock, covered, for 4-6 hours, or even longer if you have time, topping up with water if necessary. Six hours will give you a good strong stock to work with.
At the end of cooking time, strain the stock of all solid ingredients and discard them.
To reduce the stock, pour the liquid stock back into the pot and turn the heat up to medium. Cook until the liquid is reduced by half, which will take about one hour, perhaps longer. The further you reduce the stock, the stronger and more intense it will be. The stock can be used as is, or can be reduced further or frozen for future use.