This sauce is delicious served over pasta, couscous, or with rice and beans.
Place oil in a large stockpot set over medium heat. Add garlic and onion; saute until soft, about 6 minutes. Add carrots, squash, zucchini, salt and pepper to taste, rosemary, and thyme. Stirring occasionally, cook until vegetables are soft, about 15 minutes.
Transfer half the mixture to a bowl; set aside. Gently crush the plum tomatoes with your hands, and add them (with juice) to the pot. Simmer over medium-low heat until sauce begins to thicken, about 1 1/2 hours. Add reserved squash mixture, and heat. Season with salt and pepper. Serve.
The sauce can be frozen in a container for up to 2 months.