Place a fine wire mesh strainer over a blender and strain the mixture from the bottom of the slow cooker. Add half of the cooked onion and roasted garlic Blend until pureed.
Transfer gravy mixture to a small pot over medium heat. Mix 2 tablespoons of cornstarch with 2 tablespoons of water. Add to saucepan, increase heat and bring to a boil, stirring until thickened.