Caramelize the ingredients: In a large pot, heat the oil over medium-low heat and sauté the chopped onions until caramelized. Add garlic until lightly browned, then stir in chopped tomatoes and diced zucchini. Cook for 5 to 8 minutes until softened.
Add the spices and simmer: Mix in tomato paste, thyme, oregano, basil, a chicken stock cube, and pepper. Simmer for 20 minutes.
Blend the soup: Use an immersion blender for a smooth texture for about 2 minutes; for a chunkier soup, blend for 1 minute. Remove stringy celery bits and enhance with Parmesan or Grana Padano cheese and croutons. Flavors improve after a day or two.