May I reheat food by placing it on the radiators spread around the house?

Yes you may because it is not the normal cooking method. [1] It is comparable to placing cold food on top of a pot that is already on the fire. [2] However, only dry, cooked food may be reheated; a liquid, or a solid with gravy is forbidden to reheat.

What is the difference between reheating a solid and a liquid?

Although both are fully cooked, the halacha is that a cooked solid may be reheated in the permitted manner because ein bishul achar bishul (there is no re-cooking a solid). As per liquids we say that yesh bishul achar bishul – a cold liquid is prone to re-cooking. One of the reasons is because heat is a major element of a liquid thus a cold liquid is considered uncooked.

Is it permitted to steep cold, cooked chicken inside cholent in my plate?

As far as cooking goes, even if the chicken would reach the heat of yad soledes bo [3] it is not a problem because as we said there is no re-cooking a solid, but even as far as hatmana goes, there is no issur to  enwrap a food within a food. [4] As far as fully immersing a cooked egg within hot water a shaila should be asked.

What is the halacha with regard to immersing the kishke inside the cholent?

We find a few opinions in the poskim regarding this matter. See the footnote. [5] HaRav Shlomo Zalman Auerbach ztz”l held that it is permitted since hatmana is usually done for the sake of storing heat, and without enwrapping the food the heat would be lost. Here the kishke is not enwrapped in the cholent for the sake of storing heat; rather it is there to get heated by the flame beneath the pot just as the cholent is heated by the flames. [6]

Others hold that the kishke or rice should be slightly raised above the cholent, which will satisfy all opinions. [7]

Definitions

Before we can deal with the following questions, these terms must be defined:

A k’li rishon (the first k’li) – the cooking pot, whether on the fire or off, if it is yad soledes bo it cooks whatever is placed inside.

A k’li sheini – the utensil into which the contents of a k’li rishon is poured.

A k’li shlishi – the utensil into which the contents of a k’li sheini is poured.

Is one permitted to put chalah or matza into a plate of soup?

The problem involved is bishul achar afiya – cooking after baking or roasting. The Shulchan Aruch [8] mentions two opinions as to whether it is permitted to cook [9] a baked food and concludes that there are those who permit it. Does the Mechaber mean that it is permitted even in a k’li rishon (which is off the fire) or only in a k’li sheini? It is a machlokes and one’s Rav should be asked.

As for the Rama, he says that the minhag is to be stringent and bread should not be placed even inside a k’li sheini.

The M”B 45 says that even though a ladle is classified l’chumra [10] as a k’li rishon, if the soup was served from the k’li rishon with a ladle, [11] there is room for leniency and one may put bread and matza into the plate.

Therefore the answer is: Sephardim should ask their Rav if bread may be put into a k’li rishon or only into a k’li sheini. Ashkenazim may only place bread in a k’li shlishi, [12] and in this instance a ladle is classed as a k’li sheini, rendering the plate a k’li shlishi.

Accordingly, a biscuit may not be dunked into tea made in a k’li sheini.

I like putting soup nuts (soup almonds) in my soup, is there anything I must be aware of?

Soup nuts, unlike bread, are fried, which is equivalent to cooking. Since the halacha is that ein bishul achar bishul – once a food has been cooked it is permitted to place it again in a k’li rishon (that is off the fire), one may add soup nuts to one’s soup. One may also put cooked lokshen into a pot of boiling soup (that is off the fire). Croutons however, are baked, and will have the same rule as bread mentioned in the question above.

Is one permitted to make Nescafe in a k’li sheini?

Nescafe, or instant coffee, is first brewed and then either freeze dried or spray dried. It should therefore be permitted to be put even into a k’li rishon (off the fire) following the above mentioned rule that it is permitted to cook a cooked food. However, some poskim say a cooked soluble has a status of a liquid and as such it is forbidden to re-cook it. Therefore, the M”B [13] concludes that it should only be put into a k’li sheini (although there are others who will only make it in a k’li shlishi, for various reasons).