Gruel was the staple food of the ancient Greeks, for whom roasted meats were the extraordinary feast that followed sacrifice, even among heroes, and “In practice bread was a luxury eaten only in towns”. Roman plebeians “ate the staple gruel of classical times, supplemented by oil, the humbler vegetables and salt fish”, for gruel could be prepared without access to the communal ovens in which bread was baked. In the Middle Ages the peasant could avoid the tithe exacted, usually in kind, for grain ground by the miller of the landowner’s mill by roasting the grains to make them digestible, and grinding small portions in a mortar at home. In lieu of cooking the resulting paste on the hearthstone, it could be simmered in a cauldron with water or, luxuriously, with milk.
Source: Gruel – Wikipedia