Cut onions, carrots, celery, bell peppers, and other vegetables into bite-sized pieces.
Heat olive oil in a pot and sauté the onions.
Add the remaining vegetables and sauté for 2-3 minutes.
Add tomatoes (or canned tomatoes) and mix together.
Add water and a pinch of salt, then bring to a boil.
Simmer over medium-low heat for 20-30 minutes until the vegetables are soft.
Season with pepper to taste.