Generally, one should not make a blessing on Eruv Tavshilin if all cooking was done on Erev Yom Tov and only warming or lighting candles occurs on Yom Tov for Shabbat, due to a dispute among Poskim and the principle of 'Safek Brachot Lehakel' (doubtful blessings are to be lenient). However, a recommended practice to ensure a blessing is to place raw eggs in a hot pot on Erev Shabbat (Yom Tov Sheni), as this constitutes cooking on Yom Tov for Shabbat, thereby resolving all doubts. This applies especially if one intends to insulate pots on Shabbat, which some opinions consider forbidden without an Eruv Tavshilin. The text elaborates on various opinions regarding warming food, lighting candles, and the act of insulation (hatmana), citing Rishonim and Acharonim such as the Rif, Rambam, Tosafot, Rosh, Maran HaShulchan Aruch, Chazon Ovadia, Sdei Chemed, Divrei Binyahu, Chashev HaEphod, Maharam Ben Chaviv, Yabia Omer, Or LeTzion, Kuntres Ohel Sarah, and Sha'arei Yosher, discussing the nuances of 'bishul achar bishul' (cooking after cooking) and the application of 'safek brachot lehakel' in cases of double doubt. It notes that while Sefardim might be able to bless when warming foods with gravy, Ashkenazim require further clarification.
