There is a Halachic problem covering a pot of non-fully cooked food on the Blech. Covering the pot enhances and speeds the cooking process. Even though the food would have eventually become cooked, it is prohibited to augment the process; it may well be an Hiyuv Hatat-a Torah prohibition. Covering the pot is comparable to the prohibition of "Megis"-stirring, which also enhances the cooking. Therefore, if one removes the cover to check the food and discovers that it is not fully cooked, he may not put the cover back on the pot.
Even regarding fully cooked food, there may be an issue in removing the cover. The steam creates condensation which gathers on the lid and would then fall back into the cooking pot when removed. Some would argue that this is considered cooking because the condensation immediately cools off in the air before falling into the hot contents.
Hacham David, in his Halacha Berura, rejects this approach for several reasons and does not regard that as cooking. First, it is not his intent to cook these droplets. Even though it is inevitable, he does not care about it whatsoever. Moreover, these droplets are already cooked and have merely cooled down. In such a case, the Rambam holds that "En Bishul Ahar Bishul BLach"-there is no problem recooking cooled liquid, and his minority opinion can also be factored in. Also, these droplets are so minute that they dont constitute the minimum Shiur (measure) of cooking. Finally, not all agree that the droplets which fall from the lid, which is also a Keli Rishon, cool off and lose their status. For all these reasons, Hacham David permits removing the lid.
Hacham David continues that it is more problematic to replace the lid, since in the meantime, the drops have certainly cooled off, according to all opinions. Therefore, one of the lenient factors is no longer present. He concludes that nevertheless it is permitted to replace the cover and shake off the drops of condensation before returning the lid to the pot. However, it is praiseworthy to completely dry the cover before returning it.
SUMMARY
It is completely prohibited to place a cover on a pot of not fully cooked food.
It is permitted to remove a pot cover in all circumstances. If the food is fully cooked, he may return the cover, preferably shaking the condensation off the cover before he returns it. If he wants to be stricter, he should dry it totally.
