The Rama (Rabbi Moshe Isserles of Cracow , 16th Century in Siman 259) writes that it is forbidden to seal the door of an oven on Shabbat. 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 halacha is placing the cover on a pot of hot food on Shabbat. Hacham Ben Sion writes in Ohr Lesion (Collection of responsa by Hacham Ben Sion Abba Shaul, one of the leading Sephardic Rabbis in 20th-century Israel.) 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.
SUMMARY
1. It is forbidden to place the cover on a pot of food that is not fully cooked while the pot is on the blech.
2. If the food is fully cooked or the pot has been removed from the blech, it is permitted to place the cover on the pot.
