Add the leek, carrot, brown onion, celery heart, cloves, peppercorns, bay leaf and oregano to a large casserole pan.
Fill the pot with water to cover the vegetables, leaving around 5 centimetres space at the top of the pan.
Place the lid onto the saucepan and bring to a boil. Boil for 5 minutes and then reduce to a simmer for 3 hours.
After 3 hours turn off the heat. Remove the leek, carrot and onion and discard. Place a seive over the top of another large casserole pan or mixing bowl. Carefully pour the stock through the seive to strain.
Remove the seive and use a heat proof measuring jug to divide stock between clean glass jars. If freezing the stock, make sure to leave around 2-3 centimetres (depending on the size of the jar) of space at the top of the jar. This is to prevent the glass breaking as the stock expands when freezing. Place the lid ontop of each jar.
Write "vegetable stock" and the date onto labels and stick onto each jar before freezing or refrigerating.