May one use an egg for the Eruv Tavshilin?[1]
Yes, and so is the custom of some communities.[2] However, initially, one is not to use an egg but rather a food of significance such as cooked meat or chicken or fish.
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[1] See Siddur Admur “Tavshil Chashuv such as meat or fish”; Admur 527:3 and 527:11 [mentions meat, fish or egg]; Beir Heiytiv 527:2 in name of Shelah; Siddur Arizal; Elya Raba 527:5 in name of Shelah; Mateh Efraim 625:38; M”B 527:8; Nitei Gavriel 82:6
[2] See Admur 527:11; Aruch Hashulchan 527:13; Kaf Hachaim 527:21; Nitei Gavriel 82:8;