Can one put raw fish in a tinpan with some water snd spices on a electric platter a few minutes before Shabbos to eat at the Friday night seudah???
I am assuming that you are putting it on to an Israeli “palata” hot plate, which does not have adjustable settings. Since it does not have adjustable settings, according to most poskim, shehiya will not apply to it, and it would be permitted. You must remember though that if it isn’t fully cooked you can not open the cover of the fish to see if it is ready, because if it isn’t fully ready, you will not be able to close the cover, as that would be advancing it’s cooking. Additionally if differant part of the palata have differant heat levels, you can not move the fish from one place to another, as that would also advance it’s cooking.
If the hot plate has adjustible settings, aside from the above, you would need to put a blech on to it beforehand, and prefferably cover the settings.
Har Tzvi 136, Biur Halacha 318-3 D:H V’afilu, Rosh Yosef 31b D:H Ein, Igros Moshe O:CH4-74 Bishul-22, Shevet halevi 5-30, Minchas Yitzchok 5-91, Shmiras Shabbos Khilchoso 1-25 ftnt. 71 (old edition) in the name of R’ S. Z. Auerbach zt”l. Also see Shvus Yitzchok Chap.8-1.