Cooking from Yom Tov to Shabbos and the definition of ofeh

Based on his chiddush that the chiyuv of ofeh is not when the dough is placed in the oven but only after it bakes, the Afikei Yam raises an interesting idea to resolve a question of the Magen Avraham. The gemara in Pesachim (46b) quotes a machlokes Rabbah and Rav Chisda whether one is chayav malkos for cooking on a Yom Tov for a weekday. Everyone must agree that it is permissible m’doraysa to cook from a Yom Tov to Shabbos because otherwise our eiruv tavshilin would never work – an eiruv would have no power to suspend an issur d’oraysa of cooking. The question is what that heter is based on. Rav Chisda’s approach is that “tzorchei shabbos na’asin b’yom tov” – there is no issur of cooking for Shabbos on Yom Tov, which some rishonim explain to mean the 2 days are “kedusha achas”, they share the same kedushas hayom and are like one unit. Rabbah, on the other hand, argues that there is never an issur to cook for the next day, be it Shabbos or chol, because “ho’il v’ey miklaey lei orchim”, since guests may arrive who need that as a meal, the cooking is not defined as done for tomorrow’s needs, but for the needs of the Yom Tov day itself. The Magen Avraham very practically writes that therefore if one wishes to cook on Yom Tov for Shabbos, one should do so early in the day when there is a potential for that food to be needed for and used by guests. He is surprised that the practice does not appear to conform to his conclusion – people do cook late in the day! Of course, there is always Rav Chisda’s opinion to rely on, but why get involved in an unresolved machlokes? The Afikei Yam offers a novel suggestion. If one assumes that the chiyuv for baking (and cooking follows suit) follows the completion of the act rather than its start, then food placed in the oven late in the day has an advanatage. One can never be chayav for melacha on Yom Tov, because the cooking did not finish on that day. At the same time, one is not chayav on Shabbos, because the act merely completed on Shabbos and one is never chayav for an act’s completion without it having been started also on Shabbos. I’m not sure practically whether all cooking late in the day conforms to the Afikei Yam’s idea, but it is a creative lomdus!