Heat the oil in a deep pan over medium heat. Add the onions, garlic, and sauté for 2 minutes. Add the bell pepper, zucchini, yellow squash, mushrooms, diced tomatoes, stock, spices, herbs, and bring to a boil. Reduce heat and simmer until the vegetables are tender. Pass through a sieve and return the liquid to the pan. Reduce over high heat to concentrate the flavors and adjust seasoning. If needed, thicken with a little cornstarch water mixture. Return the vegetables to the reduced liquid and serve immediately.