For vegetable stock, heat 50ml oil in a large, heavy-based pot. Over medium heat, sauté 500g cubed fresh carrots, celery, onion and leeks for about 10 minutes, taking care that they do not burn. Pour in 2,5 litres of water. Place 2 fresh bay leaves, 2 cloves, 3 black peppercorns, a sprig of fresh thyme and 1 peeled garlic clove in a muslin cloth and tie with kitchen string. Add this parcel to the vegetables. Simmer partially covered for about 1 hour. Stir every 25 minutes and top up with boiling water as it reduces. Strain through a fine sieve, discard the vegetables and muslin parcel. Pour into containers with tight-fitting lids, but allow to cool before freezing.
For beef stock, roast 500g lean beef bones on a baking tray until brown, about 30 minutes. Add these and 1 x 410g tin tomatoes to the vegetable stock at the simmering phase. Skim fat off the surface as it cooks.