If someone before Shabbos filled a thermos with hot sweet tea, can that tea be used as TEA essence on Shabbos with hot water? The tea in the thermos stays hot / warm for a while through shabbos.
According to the Ashkenazi minhag the tea in the thermos is no different than any other tea that was made before Shabbos. If the tea is still warm enough that you would drink it that way then it is permitted to even pour directly from the urn on to it. If the tea has cooled to the degree that you wouldn’t drink it that way because it is not hot, then it is considered cooled (according to the Ashkenazi Minhag) then the tea should be added preferably to a kli sheishi before the hot water was poured into it, however it can also be poured directly on to the water of a kli sheini.
There is no issue of hatmana in a themos as it is a kli sheini.
M:B 318-39, Shulchan Aruch O:CH 257-5