Heat 1 1/2 tablespoons of the olive oil in a large pot over medium low heat. Add the onions and garlic and cook until soft.
In a large frying pan, heat 1 1/2 tablespoons 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 frying pan each time you add a new set of vegetables. Once each batch has been sauteed add them to the large pot as before.
Season with salt and pepper. Add the bay leaf and thyme and cover the pot. Cook over medium heat for 15 to 20 minutes.
Add the chopped tomatoes and parsley to the pot and cook another 10 to 15 minutes. Stir occasionally.
Remove the bay leaf and thyme sprigs and adjust seasoning before serving.