Prepare vegetables by cutting onions into wedges and roughly chop carrots and celery. Drizzle oil into pot over medium to high heat. Add onions, carrots, celery and crushed garlic cloves. Stir and cook for 15-20 minutes until vegetables caramelize and are browned; lower heat to prevent burning. Add remaining ingredients (use just mushroom stems if available) and bring to a boil. Lower heat to simmer and cook covered for 45 minutes. Strain and discard vegetables. Chill and store cold in the refrigerator or freeze to use later.