Heat oil in a frying pan over medium-high heat. Add the onion and saute until translucent, about 3 minutes. Lower the heat to medium and add the garlic. Cook until fragrant, about 2 minutes.
Rremove from heat and transfer to a 5-quart slow cooker. Add all the remaining ingredients and stir to combine.
Cover and cook on low for 8 hours or high for 4 hours.