Consider the second order elliptic PDE 
$$
\begin{cases}
Lu = f &\text{ on } \Omega\,,\\ u=0 &\text{ on } \partial\Omega\,,
\end{cases}
$$
where $f\in C^\infty(\Omega)$, $\Omega\subset\mathbb{R}^2$ is bounded and convex and $\partial\Omega$ is smooth. What are the conditions needed to be put on $L$ and $f$ such that the solution $u$ only has one critical point? I am looking for a result similar to Theorem 2 of [this paper][1](ScienceDirect):

> **Theorem 2:** Let $\Omega$ be a bounded, strictly convex domain in $\mathbb{R}^2$ and $u\in C^3(\Omega)\cap C^1(\bar{\Omega})$ a solution to the boundary value problem
\begin{align*}
\Delta u &= f(u,\nabla u)\quad\text{ in }\Omega\,,\\
u=\text{const}\,,& \quad\nabla u\neq0\,,\quad\text{ on }\partial\Omega\,,
\end{align*}
where $f\in C^1$, $f_u\geq0$. Then $u$ has exactly one critical point in $\bar{\Omega}$ and there $\det(D^2u)>0$ holds (i.e., a global maximum or minimum).

I think possibly that if $\nabla f$ is never $0$ that we have a similar statement, but I am not sure. 

  [1]: https://www.sciencedirect.com/science/article/pii/S0022247X83711364