Gruel is basically a thin porridge or soup. The main forms of gruel include rice gruel, flour gruel and millet gruel. Other base ingredients you can boil include breadcrumbs or ground crackers. You can best understand it by making it for yourself. Here’s a simple recipe for flour gruel: 2 teaspoons of flour, 1 teaspoon of salt. Boil one cup water. Separately, drip water on flour and salt until it makes a paste. Add the paste to the boiling water. Stir to a semi-fluid consistency. Strain to eliminate film. Serve warm. Link.