We will begin our discussion with a fascinating machloket about cooking bones between two of the great poskim of the previous generation, Rav Moshe Feinstein and Rav Shlomo Zalman Auerbach.[2]
Rav Feinstein[3] writes that the requirement that food that one wants to return to a heat source must be fully cooked does not apply to bones, as they are not considered food. He relies both on logic/observation (people do not eat bones) and on halachic sources (bones do not have a halacha of meat with regard to the laws of kashrut[4]). Therefore, making bones soft enough to be theoretically fit for consumption on Shabbat is not considered cooking.
Rav Auerbach – who lived in Israel, where it is much more common to eat at least some bones – maintains that bones are considered food with regard to the laws of Shabbat, even if they are not equivalent to meat in the context of kashrut.[5] Accordingly, he says that one must make sure that the bones are fully cooked before doing anything that will hasten the cooking (e.g., returning them to the fire, moving them to a hotter part of a blech, or returning a pot cover that one had removed).
Since Shemirat Shabbat K’Hilchata was written by a close disciple of Rav Auerbach, it is no surprise that he is stringent on the matter. However, as we have seen, this opinion is not unanimous. Indeed, these two halachic giants argued to an extreme. Rav Feinstein maintains that even an individual who eats bones can have them cook on Shabbat; his nonstandard behavior does not turn bones into food. Rav Auerbach asserts that, on the contrary, even one who does not eat bones must be careful not to cook them; they are considered food, as many people do eat them.
We are torn as to which approach to accept. On the one hand, Rav Auerbach’s general logic is compelling (halachic details are beyond our present scope). On the other hand, there are crucial indicators supporting the view of Rav Feinstein: 1) It has long been common practice to ignore the bones’ status and 2) there are no explicit earlier sources on this frequent occurrence.[6] Rav Auerbach himself does not take his thesis the whole way: Although one is not permitted to increase the heat on food if there is a doubt regarding whether it is fully cooked,[7] Rav Auerbach permits those who do not plan to eat the bones to move food with bones to a hotter place.
In addition to the assumption that bones are not food, there are two other basic reasons for leniency:[8] 1) One’s intention is focused on the meat (keep it hot), rather than the bones (finish their cooking). 2) In many cases, it is possible that the bones are already considered cooked. (It is difficult to make this determination, especially considering that there is a machloket regarding whether cooking is forbidden after a food has reached the stage of ma’achal ben d’rusai.[9])
We would certainly not criticize one who ignores the issue of bones, at least if members of his household do not generally eat bones. Even if you want to be machmir and follow the view of Rav Auerbach, you can be lenient in your case, in which the stew has been cooking for a long time (albeit on low heat). In that situation, any bones that are not ready at night are also not the ones that you (and presumably most others) would eat. Regarding such bones, Rav Auerbach presumably agrees with Rav Feinstein that the halachot of cooking do not apply.