How liquid does something have to be to become prohibited from reheating on a blech on Shabbas?
I'm really wanting to know how "soft" or mushy something can be before being considered liquid and not permitted but that's harder to describe.
I asked this exact question to Rabbi Shlomo Mordechai Breslauer, rav of Bais Tefilo in Monsey, NY.
He told me to put the substance on a surface that is not slippery (e.g. a paper plate), and hold it at a 45° angle. If the substance rolls over itself, then it is considered a solid. If if pours, it is considered a liquid.
My rav explained a simple rule:
"If you can pick it up with a fork, it's solid. If you can't and you need a spoon, it's liquid."
Based on this idea, it seems that the gelled chicken gravy is OK. So is p'tcha, but who wants to eat that heated up, anyway?
Shulchan Aruch Orach Chayim siman 318, sayif 4:
וה"מ שיש בו בישול אחר בישול, בתבשיל שיש בו מרק, אבל דבר שנתבשל כבר, והוא יבש, מותר לשרותו בחמין בשבת.
Regarding a meal with soup ...
ומ"מ אם הוא תבשיל שיש בו רוטב ומצטמק ויפה לו אסור לדברי הכל:
meal with sauce ...
דבר שנתבשל כל צרכו והוא יבש שאין בו מרק
... Dry without soup ...
Magen Avraham on Sayif 16 sk 40, about fatty food.
כדי שלא יהיה נימוח כל כך שיזוב לחוץ אלא יתנו מרחוק ואף שנימוח בתוכו ל"ל בה כמ"ש הרב"י וכ"כ בספר מ"כ עיין שם וכן משמע בהגהות סמ"ק דצלי ופשטי"ד שוין:
The problem is when the fat is runny (for the poskim who are stringent about fatty foods).
Source Mishna & Gemara Shabbat 146b:
WHATEVER WAS PUT INTO HOT WATER BEFORE THE SABBATH MAY BE STEEPED [AGAIN] IN HOT WATER ON THE SABBATH;
Rashi " כדי שיהא נמוח " stepped to moisten and tenderize it. The literal translation is to make it melted. Poskim inferred. Not melted == Dry. Gemara
What, for example? R' Safra said: E.g. , R' Abba's fowl[s]. R' Safra also said: I once paid a visit there [Palestine] and ate thereof, and but for R' Abba who made me drink wine of three foliages I would have been in danger.
בדבר לח וצלול לדברי הכל יש בישול אחר בישול
Something moist and fluid has unanymously a prohibition of bishul after bishul.
The makor in poskim is Rabenu Yerucham See right top of th page.
Liquid is liquid. Shmirat Shabat Kehilchata allows putting chicken on a blech, even though you know that the liquids inside will take a liquid form outside, due to the (relative) heat (using something to separate the utensil from the Blech).
By the way, according to Rambam and there are Teimanim who follow his Psak, There is no second bishul for liquids, so they preheat water before Shabos, and put some of it on the blech as needed.