Halacha forbids deriving benefit from a Melacha (forbidden activity) performed on Shabbat. Thus, if a person cooked food on Shabbat in violation of Halacha, it may not be eaten.
This prohibition was enacted by the Sages. As far as Torah law is concerned, food that was cooked on Shabbat is permissible for consumption; it was the Sages who enacted this provision prohibiting eating food that was prepared in violation of Shabbat.
The Rabbinic origin of this prohibition yields important ramifications. Hacham Ovadia Yosef ruled that if food was prepared on Shabbat in violation of Halacha, but there are authorities who permitted food preparation in that fashion, then the food is permissible for consumption. Even though Halacha does not follow that opinion, and forbids performing the act in question, nevertheless, if the act was done, the food may be eaten. Since the prohibition against eating food cooked on Shabbat constitutes a Rabbinic, rather than Biblical, prohibition, it does not apply in situations of Safek (Halachic uncertainty). As such, if there is some question among the Halachic authorities as to whether the food was in fact prepared in violation of Shabbat, the food may be eaten.
One example of this rule is the case of food that had been half-cooked before Shabbat, a situation known in Halacha as "Ke’ma’achal Ben Derusai." According to some authorities, if a food had been half-cooked before Shabbat, one who cooks that food on Shabbat has not violated Shabbat. Since the food was already edible, at least for some people, completing the cooking process does not violate the Shabbat prohibition against cooking. However, the Shulhan Aruch does not follow this view, and rules that fully cooking this kind of food constitutes a violation of Shabbat. Therefore, if a food had not been fully cooked before Shabbat, one may not put it on the stove or on the Blech on Shabbat to complete the cooking process.
Nevertheless, if a person did place half-cooked food on the Blech to finish cooking, the food is nevertheless permissible. Since there are authorities who permitted cooking food in this fashion, the food may be eaten despite the fact that it was cooked in violation of the accepted Halacha. Thus, for example, if a person is a guest at somebody’s home on Shabbat, and the host mentions that the food being served had only been half-cooked before Shabbat, and it was placed on the Blech in the morning to finish cooking, he may eat the food. Even though the host acted incorrectly by placing the food on the Blech, it is nevertheless permissible.
Another example is soup that was reheated on Shabbat. The Shulhan Aruch rules that one may not reheat liquid food on Shabbat, even though it had been fully cooked before Shabbat. Therefore, one may not take Kibbehamda (soup with meatballs), for example, out of the refrigerator on Shabbat morning and put it on the Blech so it can be served during lunch. Nevertheless, if somebody did place a pot of soup on the Blech on Shabbat morning, it may be eaten. The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) held that reheating liquid food is permissible on Shabbat, because, in his view, cooking any food that has been cooked before Shabbat does not violate the prohibition of "cooking" on Shabbat. Halacha does not follow this opinion, and forbids reheating liquid food on Shabbat. Nevertheless, if one did reheat liquid food, the food may be eaten, in light of the different opinions on the subject.
Hacham Ovadia (listen to audio recording for precise citation) also applies this rule to a case of raw food that one cooked during the period of Ben Ha’shemashot (twilight) on Friday afternoon. The term Ben Ha’shemashot refers to the 13.5-minute period immediately after sundown, and it is uncertain whether this period should be considered daytime or nighttime. It is therefore clearly forbidden to cook during this period, as one thereby may be violating a Torah prohibition. However, food that was cooked during the period of Ben Ha’shemashot may nevertheless be eaten. This food has the status of Safek; there is a possibility that it was cooked on Shabbat, but there is also a possibility that it was cooked before Shabbat. Therefore, this food may be eaten, since it is uncertain whether or not it had been cooked on Shabbat. It must be emphasized, however, that it is certainly forbidden to cook during Ben Ha’shemashot; this discussion refers only to a case of someone mistakenly cooked during this period, in violation of Halacha.
Summary: Generally speaking, food that was cooked on Shabbat in violation of Halacha may not be eaten. There are, however, several exceptions: 1) If food was half-cooked before Shabbat, one may not put it on the fire or Blech to finish cooking on Shabbat, but if he did, the food is permissible. 2) One may not reheat soup or other liquid foods on Shabbat, but if one did reheat liquid food that had been cooked before Shabbat, it may be eaten. 3) One may not cook during Ben Ha’shemashot (the 13.5-minute period after sundown) on Friday afternoon, but food cooked during this period may be eaten.
