Preheat the oven to 200°C.
Chop the bones into manageable pieces and place them in a large roasting tin.
Add halved onions, halved garlic, and 1.5 cm of water to the tin.
Roast bones and vegetables for 40 to 45 minutes, topping with water if necessary.
Fill a large stockpot with 4 to 5 litres of cold water and bring to a simmer.
Add roasted bones, vegetables, and browned bits from the tin to the simmering water.
Add the bouquet garni, leeks, carrots, celery, tomatoes, salt, and peppercorns.
Simmer gently while covered for 4 to 6 hours.
Strain the liquid into a clean pot and discard all solid ingredients.
Boil the liquid over medium heat for about one hour until reduced by half.