Let $R,S,T$ be commutative rings and assume that $R,S$ are $T$-algebras.
In an answer to this question, Pierre-Yves Gaillard gives an example of an $R \otimes_T S$-module that cannot be written as the tensor product of an $R$-module and an $S$-module (there, $T=k$, $R=S=k^2$ where $k$ is a field).
I'm interested in the relation between the module categories Mod-$R$, Mod-$S$ and Mod-$R \otimes_T S$. Is there some kind of general operation (a "tensor product") on abelian categories that takes Mod-$R$, Mod-$S$ and Mod-$T$ (or $T$ itself) and produces Mod-$R \otimes_T S$?