Heat the oil in a large saucepan. Add the onions, peppers and garlic. Cook at a medium heat for about 5 minutes. Stir often to ensure they do not stick to the bottom of the saucepan.
After 5 minutes add the zucchini, eggplant and mushrooms. Continue to cook for about 8 minutes, stirring often.
Stir in the tomatoes, herbs and spices. Lower heat to a simmer and cook for another 30-40 minutes, stirring occasionally. The sauce should be reduced and thickened.