Is making cholent on Shabbos a minhag that is so universal that it has in effect become required for Ashkenazim?
Or, if I am correct that it is required (by at least some authorities) to have hot food for one's daytime Shabbos meal, is simply heating up some solid cold food on a warming plate or blech sufficient to fulfill this requirement?
Yes. And no.
Cholent is said to have evolved of a tradition to have hot food on Shabbath.
The basis of this tradition seems to be a rabbinic insistence, possibly even a decree, that eating hot food shows that a person is not a karaite, and instead accepts the validity and authority of Torah SheBe'Al Peh.
The fear that someone rejected TShB"P was apparently serious enough that the Geonim and certain later rabbis stated explicitly that if someone absolutely refused to eat hot food on Shabbath, he is to be suspected of karaism.
אנציקלופדיה תלמודית כרך ט, הטמנה [טור א]
ב. מצותה. הטמנת חמין - באופן המותר, היינו בדבר שאינו מוסיף הבל - תקנה גדולה היא שתיקנו חכמים משום עונג שבת, שהרי כל ימות החול אוכל אדם תבשיל חם, ותיקנו חכמים לטמנו מבערב כדי שישתמר המאכל בחמימותו ויהיה חם בשבת, וכל מי שאינו אוכל חמין בשבת דרך מינות יש בו51, והפליגו הגאונים שבן נידוי הוא וצריך להפרישו מקהל ישראל52, אבל מי שמזיקים לו החמין מותר לאכול צונן53, ויש לנו לדקדק על מי שאינו אוכל חמין בשבת, אם משום חולי הוא או שאינו תאב לאכול מאותו מאכל, ולא משום צד מינות, אין לנו להוכיחו על כך, אבל יש לזרזו ולהודיעו להתרחק מאותו דרך ולהכריח עצמו שיענג את השבת כדרך שתיקנו חכמים בהטמנת חמין54.
Encyclopedia Talmudit, Volume 9, Insulating [Sec. 6, point 2]
B. The Mitzvah. Insulating Hamin – in the permitted way, that is in something that does not increase steam - is a great requirement, which the Sages decreed for the sake of enjoyment on the Sabbath, for on every day of the week one eats hot food, and the Sages decreed that one insulate from Friday, so that the food will maintain its heat and be hot on the Sabbath. And as for all who do not eat Hamin on Shabbat, there is a hint of heresy in that, as the Geonim warned that [the one who does not eat Hamin on the Sabbath] is an apostate and should be removed from the Jewish community. But one who regularly eats Hamin is permitted to eat cold [food]. We should scrutinize one who refuses to eat Hamin on the Sabbath; if it is because of an illness or that he does not enjoy that particular food, and not because of heresy, we should not rebuke him for this, but we should educate him and inform him that he should distance himself from this path and try to get used to enjoying the Sabbath in the way that the Sages decreed, with insulating Hamin.
אנציקלופדיה תלמודית הערות כרך ט, הטמנה, טור ו הערה 51
תשוה"ג שערי תשובה סי' לד; ס' העתים סי' טז; המאור שבת פ"ג: צריך בדיקה אחריו אם הוא מין; ארחות חיים שבת סי' עב; כלבו סי' לא; אבודרהם סוף שחרית של שבת; רמ"א בשו"ע סו"ס רנז; מצוה להטמין כו'.
Above, footnote 51
An investigation is required into him, [to determine] if he is a heretic. (Responsa of the Geonim Sha'arei Teshuvah 34; S. Haitim 17; Hamaor Shabbat 83; Orchot Chaim Shabbat 72; Kolbo 31; Abudraham, end of Shabbat morning service; RaM"A on Shulchan Aruch, end of 257).
אנציקלופדיה תלמודית הערות כרך ט, הטמנה, טור ו הערה 52
- תשוה"ג שם, ועי' הערות לס' העתים שם שהוא לפי שנתפרצו בזמנם הקראים שאסרו ההטמנה.
Ibid, footnote 52
This is because some [people] defected at the time of the Karaites, who forbade insulation.
תלמוד בבלי מסכת שבת דף לד עמוד א
משנה. שלשה דברים צריך אדם לומר בתוך ביתו ערב שבת עם חשכה: עשרתם, ערבתם? הדליקו את הנר. ספק חשכה ספק אינו חשכה - אין מעשרין את הודאי, ואין מטבילין את הכלים, ואין מדליקין את הנרות. אבל מעשרין את הדמאי, ומערבין וטומנין את החמין.
Babylonian Talmud, Tractate Shabbat, 34b
Mishnah: Three things a man must say within his house on Sabbath Eve (Friday) as dusk approaches: ‘Have you tithed? Have you made an Eruv? Light the candle(s).’ If there is doubt whether dusk has settled or not – do not tithe the certain, and do not immerse the vessels, and do not light the candles. But tithe the uncertain, make the Eruv and insulate the Hamin.
As a result, it seems, different communities developed traditions and recipes that descended through the generations for the official hot Shabbath dish. The word cholent, according to some, may actually derive from the French (why French, I'm not sure; the Spanish word is caliente. Perhaps old French had a closer cognate than modern French) word for hot. The word used in Halachic literature is Hamin, which is the rabbinic Hebrew word for hot; this also happens to be the word commonly used among non-Ashkenazic Jews, and in Israel.
From this slight ambiguity, however, it's not entirely clear whether, or indeed even why, one would have to eat a particular type of hot dish, or even whether/why one would be required to have a hot food every week, so long as it's clear that said person does not object, and indeed regularly prepares and consumes something hot on Shabbath.
For what it's worth, I've heard many rabbis talk about there being no need for it to contain meat, barley, beans, potatoes, or any one particular ingredient commonly associated with cholent.
From the Rama 257:8
ומצוה להטמין לשבת כדי שיאכל חמין בשבת, כי זהו מכבוד ועונג שבת. וכל מי שאינו מאמין בדברי החכמים ואוסר אכילת חמין בשבת, חיישינן שמא אפיקורוס הוא (הר"ן פרק במה טומנין וכל בו):
Loose translation: it is a Mitzvah to have heated food on shabbas because this a honor and pleasure on shabbas,and anyone who does not believe in the words of the Chachamim and prohibits hot food we should be concerned maybe this person is a apikoris(heretic).
Rav Nebantzol's notes on the Mishna Brurah writes " maybe even if one does not like hot foods one is not obligated since the main reason is to go against the karites who don't allow fires in their homes,and it seems one fulfills this with lighting shabbas candles". So the fact you have a your stove on or even shabbas candles it should be fine. Warming up seems to be just as good.
Short answer:
It is absolutely OK to not have cholent.
Some authorities require one to have some hot food.
Most authorities require that you not refuse or avoid hot food systematically.
That said, it is a widespread issue that there are those who "know" it is required to have cholent, and will think you strange (or worse) if you don't. Some may even refuse to allow their children to marry you(r children).
Rabbi Leibel Katz in Ohel Areyh siman 216 writes that one could fulfill eating hot food on shabbas by drinking a tea or coffee but one should certainly have a hot item since it is not just for oneg shabbas but also to remove the suspicion of being an apikoris(and chilul shabbas see Mishna Brurah).
