I start it from the end: regarding Lot it is written (Bereishit 19:3):

וַיִּפְצַר־בָּ֣ם מְאֹ֔ד וַיָּסֻ֣רוּ אֵלָ֔יו וַיָּבֹ֖אוּ אֶל־בֵּית֑וֹ וַיַּ֤עַשׂ לָהֶם֙ מִשְׁתֶּ֔ה וּמַצּ֥וֹת אָפָ֖ה וַיֹּאכֵֽלוּ׃
And he urged them greatly; and they turned in unto him, and entered into his house; and he made them a feast, and did bake unleavened bread, and they did eat.

Rashi says there that it was passover, moreover, he adds that it didn't took much time for the angels to get there from Hebron, so it can be assumed that it was erev pesach the earliest when the angels visited Abraham. This fits well with the words of the gemara in Rosh haShanah 10b–11a, where both Rabbi Eliezer and Rabbi Yehoshua say that Isaac was born on passover (and his birth was announced a year in advance). Berakhot 27a concludes that it was noon when the visitors arrived to Abraham, and Bereishit Rabbah 48:12 explicitly says that Sarah made unleavened bread for passover.

How comes then Rashi to comment on Bereishit 18:6 that

קמח סלת – סֹלֶת לְעֻגוֹת, קֶמַח לַעֲמִילָן שֶׁל טַבָּחִים, לְכַסּוֹת אֶת הַקְּדֵרָה לִשְׁאֹב אֶת הַזֻּהֲמָא.‏
meal [and] fine flour – The fine flour for the cakes [and] the meal for the starch used by cooks to cover the pot, to draw out the scum.

when it is known that this cook's dough is not allowed on passover (Mishnah Pesachim 3:1; gemara 42b)?

  • 1
    Although note that Rosh Hashana 11a seems to assume that the angels came in Tishrei
    – Joel K
    Commented Oct 25, 2021 at 16:31
  • @JoelK I haven't learned this yet, but I can't figure out why the angels' visit and the conception had to coincide in time. Commented Oct 25, 2021 at 17:17
  • 2
    The Tur HaAroch and Megaleh Amukos (on Bereishis, ד"ה יש כאן) address this question, but I don't understand their answers.
    – robev
    Commented Oct 25, 2021 at 18:19
  • See Rashi HaShalem
    – robev
    Commented Oct 25, 2021 at 20:17
  • 2
    Perhaps there is a way to make it kosher for pesach.
    – N.T.
    Commented Oct 26, 2021 at 4:31