The meat can be any mixture, I like to use a mixture of chicken wings, ox tail and chopped brisket or whatever. Trim the carrots, scrape and cut into half lengthways, Peel and cut in half the onion. Top and tail the celery.
Lightly coat the meat with a little tomato puree and put into a shallow oven proof dish, add the carrots, onion and celery and put in the oven at 400°F/200°C/Reg6 for an hour turning half way through. The aim here is to partially carmelize the vegetables and meat.
Meanwhile top and tail the leeks, take off outer leaves and cut into one inch slices. Take off outer skin of the garlic and trim bottom of the head. Tie up half a dozen stalks of thyme with some string.
When ready pour the meat, vegetables and all the liquid into a large saucepan. Add the leeks, garlic and thyme and barely cover with water. Cover and simmer for a couple of hours checking the level of liquid is OK a couple of times.
When ready use a slotted spoon and a sieve to discard all the vegetables and meat and pour the stock into another saucepan. Reduce the stock by about a third, allow to cool and then freeze for an hour or so.
When you want to use the stock, use a spoon to scrape off all the fat from the surface. Rewarm and season with salt and freshly ground pepper.