An eruv tavshilin is food that one sets aside before Yom Tov, when Yom Tov will be immediately followed by Shabbat. As long as the eruv exists, everything that one may do on Yom Tov for the sake of Yom Tov may be done for the sake of Shabbat as well.
The food used for the eruv must be the kind of food which it is appropriate to eat with bread. This includes meat, fish, eggs, and spreads (as long as they are cooked). In contrast, food that is not appropriate to eat with bread – such as cereal, noodles, and rice – is not appropriate for the eruv, even though it is cooked (Beitza 16a; SA 527:4). Many people use hard-boiled eggs for the eruv, since they can last until Shabbat even without refrigeration (AHS 527:13).
The food used for the eruv may be cooked, roasted, boiled, or smoked. Even pickled food may be used, as there is a principle that “pickled is considered cooked.” However, raw food may not be used as an eruv (Beitza 16b; SA 527:5; SHT ad loc. 25).
It is preferable le-khatḥila to set aside an egg’s bulk (kebeitza) of bread as well, as there is an opinion that an eruv’s cooked food allows people only to cook, while baked food is necessary to allow people to bake (Rabbeinu Tam). Nevertheless, technically, one who set aside only cooked food may bake as well as cook for Shabbat (SA and Rema 527:2-3).
Just as a kezayit of food is adequate for one person’s eruv, it is sufficient for a household as well. Similarly, one who is setting aside an eruv on behalf of all the city’s residents may use one kezayit for everyone (Beitza 16b; SA 527:3).
It is preferable le-khatḥila to set aside a substantial portion of food. When possible, it is a good idea to use a pot full of food that was cooked before Yom Tov for Shabbat. Nevertheless, technically, even if one takes lentils that were left on the bottom of a pot, and which had been cooked for weekday use, he has fulfilled his obligation, as long as there is at least a kezayit of them (Beitza 19a; SA 527:6; MB ad loc. 8).
Many people eat the eruv’s cooked food at one of the Shabbat meals. Since this food has been used for one mitzva (eruv), it is appropriate to use it to fulfill an additional mitzva (oneg Shabbat). Similarly, many people use the eruv’s bread as part of leḥem mishneh at se’uda shlishit (MB 527:11, 48).
Even if one ate part of the eruv’s cooked food on Yom Tov, he may still cook and bake for Shabbat, as long as a kezayit of the food remains. However, if less than a kezayit is left, no further melakha may be done on Yom Tov for Shabbat. Even if the eruv’s bread remains, it is not good enough, as the primary part of the eruv is the cooked food (SA 527:15; MB ad loc. 7).