If you want to thicken the sauce from the slow cooker, you can use one of these methods: Transfer the sauce to a small saucepan and simmer it over medium-high heat until reduced and thickened. Mix cornstarch or flour with water and stir it into the sauce. Cook until thickened, stirring constantly. Use an immersion blender to puree some of the vegetables in the sauce. This will add body and texture to the sauce.