Cut the onions and garlic into halves (do not need to peel them).
Coarsely chop the carrots, celery stalks and leeks into chunks.
Heat oil in big pot.
Add all the vegetables into pot.
Cook over medium heat for 5 –8 mins and stir frequently.
Add water and bring to a boil.
Lower heat and simmer(uncovered), for 45mins.
Stir every 5 –10 mins
Cool completely, strain with a strainer, discard vegetables.
Store in containers. Less water means that your stock will be more concentrated. Refrigerate thestock in an airtight container for up to 1 week. Freeze for up to 3 months.