It is prohibited to cover a pot of uncooked food on the Blech. Doing so speeds up the cooking process and constitutes an Issur D’oraita-a Torah violation, according to Hacham Ovadia. Therefore, if a woman took off the pot cover to check the food and discovered that it was not fully cooked, she may not return the cover to its place. If she did so, the food is forbidden to her and everyone else until Mosa’eh Shabbat, even if her action was out of ignorance..
This is based on the on the Shulhan Aruch (318:1) who rules in accordance with the opinion of Rabbi Yehuda that food mistakenly cooked in violation of Shabbat may not be eaten, neither by the offender or anyone else, until Mosa’eh Shabbat. Even though it was done “B’shogeg”-without intention, i.e. he was not aware that the action was prohibited, the Hachamim instituted a K’nas (penalty) to prevent benefiting from a violation of Shabbat.
If one mistakenly covered a pot of uncooked food on the Blech, the food is forbidden for consumption to everyone until Mosa’eh Shabbat.