Prepare onions, carrots, celery, tomatoes, and other vegetables by cutting them into bite-sized pieces.
In a large pot, add olive oil and sauté the vegetables for 3–5 minutes.
Add water or vegetable broth and bring to a boil.
Season with bay leaf, salt, and pepper.
Simmer over medium heat for 20–30 minutes until the vegetables are tender.
Remove the bay leaf and taste the soup, adjusting the seasoning if necessary.