This is the 12372nd most frequent Russian word.
капустой
The word "капустой" means "with cabbage" or "cabbage" in the instrumental case in Russian.
Here, 'капустой' is used in the instrumental case, indicating that cabbage is used as an ingredient in making the soup.
Мы готовим суп с капустой.
We are preparing soup with cabbage.
In this sentence, 'капустой' describes the filling of the pie, using the instrumental case to show the relationship.
Я купил пирог, начинённый капустой.
I bought a pie filled with cabbage.
The word 'капустой' here is used to indicate what material was used for decoration, functioning in the instrumental case.