Heat the oil** on medium heat in a stockpot. Add onions and garlic; sweat*** for 1 minute.
Add mushrooms, sweat for another minute. Add celery, carrots, and any other vegetable (if using), except tomato. Sweat for 4-6 minutes. Add tomatoes (if using). Add bayleaf, fennel seeds, and peppercorns.
Add the cold water and bring to a gentle simmer (about 20 minutes). Reduce the heat to medium-low and simmer gently for another 20-30 minutes.
Remove from heat and filter the broth through a fine-mesh sieve into a broad mouth pan for quick cooling. Once the broth is completely cooled, funnel it into bottles. You can use this now in countless recipes!