Preheat the oven to 200c. Place the bones in a large roasting tin.
Cut the onions in half (unpeeled) 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 45 minutes, topping up the water occasionally when needed.
Meanwhile, fill a large stockpot with cold water until it is about two-thirds full and bring the water up to simmering point.
When the water is simmering and the bones are roasted, add the bones, onions and garlic to the water, scraping in the browned bits from the tin.
Add the thyme, leeks, carrots, tomatoes, salt and pepper.
Gently simmer the stock, for 5-6 hours, topping up with water if necessary.
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 an hour. 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 then frozen in your ice cube tray for future use.