Reheating does not mean re-cooking. In French cooking, reheating is an art. Food must be brought back to the temperature at which it should be served without allowing any additional cooking. For some foods this can be done quickly, while for others it must be done slowly. You can reheat a soup or sauce over high heat, and serve it as soon as it comes to a boil. A large pot of stew, on the other hand, must reheat slowly, so that by the time the sauce is simmering, everything is just hot, not overcooked. In the same fashion, a rare roast beef must be reheated slowly so it will be warm when served but not cooked any more.