Heat the olive oil in a large stock pan over medium heat.
Once the oil is hot add the carrots, celery and onion. Sauté until lightly browned, about 10 minutes.
Add the tomato paste and cook for an additional 2 to 3 minutes or until browned. Stirring occasionally.
Add the water, garlic, parsley stems and mushroom stalks. Increase the heat to high and bring to a boil. Reduce the heat to medium-low and simmer for two to three hours.
Strain the stock from the vegetables into a large bowl. Let cool. Place in airtight container and refrigerate for up to one week or freeze up to three months.