Heat the oil in a large stockpot over high heat. Add the onions, carrots, celery, and garlic, and sauté, stirring occasionally, until the onions are translucent, for about 3 to 4 minutes.
Add the remaining ingredients and bring to a boil. Reduce the heat to low and simmer, uncovered, for about 1 3/4 hours. Remove from the heat, strain the stock, and allow it to cool thoroughly. Refrigerate or freeze in 2- to 4-cup containers or ice-cube trays for use in individual dishes. Keeps for 2 months.
Yield: about 2 quarts