Optional: heat the oil in a frying pan or skillet over medium heat. Add the onions. Sauté the onions till they're gently browning, about 7-8 minutes, stirring continually. Add the garlic and cook for another 2 minutes.
Transfer the onions and garlic to your slow cooker. Add the tomatoes, then crush them with a spoon or a potato masher in the slow cooker. Add the remaining ingredients. (Alternately, if you don't want to sauté the onions and garlic, begin by crushing the tomatoes in the slow cooker, then add the rest of the ingredients.)
Cook on high for 3-4 hours, or on low for 6-8 hours.** If the mixture is too thick for your liking, you can thin it with additional water. Taste and adjust salt. Serve.