Request: Proof of BPZ theorem (asymptotic mean value property and harmonic functions) – mathoverflow.net

There is a theorem, attributed to Blascke, Priwaloff, and Zaremba, that reads:

THEOREM (BPZ): Let $\Omega\subseteq\Bbb R^n$ and suppose $u\in C^0(\Omega)$. Then, $u$ is classically harmonic if and ...