Update. Changed \stackrel
to \overset
.
To get
\begin{array}{|c|c|c|c|}
\hline
x\overset{\LARGE\setminus}{\phantom{.}}\overset{\Large y}{\phantom{l}}
& 1 & 2 & 3 \\
\hline
4 & 4 & 8 & 12 \\
\hline
5 & 5 & 10 & 15 \\
\hline
\end{array}
insert $x\overset{\LARGE\setminus}{\phantom{.}}\overset{\Large y}{\phantom{l}}$ in the table top left corner, whose code is
x\overset{\LARGE\setminus}{\phantom{.}}\overset{\Large y}{\phantom{l}}
.
The table code is
\begin{array}{|c|c|c|c|}
\hline
x\overset{\LARGE\setminus}{\phantom{.}}\overset{\Large y}{\phantom{l}}
& 1 & 2 & 3 \\
\hline
4 & 4 & 8 & 12 \\
\hline
5 & 5 & 10 & 15 \\
\hline
\end{array}