Heat the remaining 1 tablespoon of oil in the skillet. Add the garlic and onion and cook over moderately high heat, stirring often, until softened, about 5 minutes. Add the tomatoes and cook, stirring, until they release their juices, about 3 minutes. Add the chicken stock and bring to a boil, scraping up the browned bits from the bottom of the skillet. Transfer the mixture to the slow cooker. Add the water.