Heat 1 1/2 tablespoon of the oil in a large pot over medium-low heat. Add the onions and garlic and cook until soft.
In a large skillet, heat 1 1/2 tablespoon of olive oil and saute the zucchini in batches until slightly browned on all sides. Remove the zucchini and place in the pot with the onions and garlic.
Saute all the remaining vegetables one batch at a time, adding 1 1/2 tablespoon olive oil to the skillet each time you add a new set of vegetables. Once each batch has been sauteed add them to the large pot as was done in step 2.
Season with salt and pepper and cover the pot. Cook over medium heat for 15 to 20 minutes.
Add the chopped tomatoes to the large pot, cook another 10-15 minutes. Stir occasionally.