Moadim L’simcha!
One important aspect of being a cohesive community is eating in each other’s homes especially for Shabbat and holiday meals. Sometimes different people in a community have different standards of kashrut, some more strict and some more lenient. Kesher Israel has a kashrut guideline so that we can be on the same page and have a common minimum standard and thus all feel comfortable eating at each other’s homes. Here is a link to the guidelines. But just to make things simple, here are a few bullet points that commonly are sources of confusion:
Checking vegetables-
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Pre-washed vegetables bought in bags in grocery stores-it is rare to find any bugs in these, (miut sheyno matzuy) checking a few pieces from the bag to rule out infestation is enough.
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Unwashed vegetables from the grocery store– wash these well or soak in a bowl of water with some vinegar for a few minutes. Check several pieces well. If you find any bugs repeat the process.
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Vegetables that are organic or bought at the farmer’s market – wash better than you would the grocery store ones, and check more thoroughly. In general using your eyes in regular light is all the halacha requires.
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Cheese-There was a rabbinic decree that for hard cheese a Jew must put the rennet in the mixture to ensure it was not made in the stomach of a non-kosher animal. Though generally in the United States today rennet is made from genetically engineered yeasts, the mainstream practice is that we still keep the decree as with other rabbinic decrees, though the reason may no longer apply. There are several hashgochas which supervise cheese to be sure the ingredients are kosher but do not require a Jew to put the enzyme into the milk. These should not be served to fellow company from the shul. (Tablet K and Ko are two such hashgochas). Use of such cheese does not render a kitchen unkosher and there were minority opinions who permitted it. Nevertheless this is not the communal standard for serving.
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Warming up precooked food on Shabbat- One should not place food in a hot oven on Shabbat since this violates the rabbinic decree against “appearing like one is cooking.” An electric shabbat hotplate (platta) is a good solution. These may be put on timers. One should not place liquids on a heat source to warm on Shabbat, only solids. Generally if the food is eaten with a fork and not a spoon it can be considered a solid. If someone did warm a liquid on Shabbat which was fully cooked before Shabbat there are minority opinions to rely on to permit this and so one should be careful as in all cases not to embarrass someone and one can eat the food.
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Placing food in a cooker, crockpot or oven before Shabbat – Food which is cooked at least half way (or a third in a pinch) before Shabbat (or uncooked food with meat in it) may be placed in an oven or crock pot or other heating device before Shabbat and allowed to stay warm or cook the rest of the way on Shabbat.
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On Yom Tov which is not Shabbat one may cook and one may light one flame from another flame (but not a new flame from a match). If one is nervous about their gas stove flame blowing out on Yom Tov (and filling the room with gas) or causing a fire, then, based on a tradition from Rav Moshe Feinstein, one may turn off the flame. It may be relit later from an existing flame. If one lit a flame to use on Yom Tov before the holiday and it blew out on its own then one may light a new flame from a match to use for cooking or for light since they did what was expected to prepare and it went out on its own (https://ph.yhb.org.il/12-05-01/) .