Heat olive oil in a large pot over medium heat.
Add the onion and cook until translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional 1 minute, until fragrant.
Add the diced carrots, celery, and bell pepper; sauté for 5-7 minutes.
Stir in the zucchini and cook for another 3-5 minutes.
Pour in the diced tomatoes with their juice and vegetable broth.
Sprinkle in the dried oregano, basil, salt, and pepper; mix well.
Bring the soup to a boil, then reduce heat and let it simmer for about 20 minutes.
Add the chopped spinach or kale, cooking until wilted, about 2–3 minutes.
Taste and adjust the seasoning before serving.