The phrase 'the subject of the lecture' is correct and usable in written English. For example, you could write: "Professor Smith gave an interesting lecture on the subject of the lecture: the history of ancient Greece.".
The subject of the lecture was "How to Meet the Right One-and Marry".
The subject of the Lecture is "related to some aspect or aspects of military surgery, medical equipment (military and civil), the surgery of children, neurosurgery, general surgery, medical ethics or medical history; these being subjects in which Downes was particularly interested".
The role of heparanase was the subject of the lecture by Israel Vlodaysky (Hadassah University Hospital, Jerusalem, Israel).
A bit of a long story but... Jo Shapcott commissioned me to write a poem for an anthology she was editing for the Royal Institute, where she was writer in residence, and the subject of each poem had to be the same as the subject of the lectures given at the Royal Institute while she was there.
Some team members pointed out that the subject of the lectures didn't relate to the problems they were facing at that moment.
(This is the subject of the first lecture, "Lightness"; the other titles are "Quickness", "Visibility" and "Multiplicity").
In England, anatomy was the subject of the first public lectures given in any science; these were given by the Company of Barbers and Surgeons in the 16th century, joined in 1583 by the Lumleian lectures in surgery at the Royal College of Physicians.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com