The Rama (a”h) writes that it is forbidden to seal the door of an oven on Shabat. In the event that food inside the oven is not fully cooked, sealing the opening amplifies the heat and accelerates the cooking process. This would constitute the prohibition of bishul.
The practical application of this halachah is placing the cover on a pot of hot food on Shabat. Ha’Rav Ben Tzion writes in his sefer, Ohr Le’Tzion, that if one removes the cover from a pot on the blech on Friday night and notices that the food is not yet fully cooked, it is prohibited to return the cover. Doing so causes the food to cook faster and is thus analogous to the case of sealing an oven mentioned by the Rama.
If the food in the pot is fully cooked, returning the cover does not enhance the cooking and is therefore permitted. It should be noted that the problem of closing the pot is only when the pot is on the blech or hotplate. If one took the pot off the fire, it is permitted to return the lid, even if the food inside is not fully cooked.
