Draw on background rectangle

I have the following code:

\documentclass[11pt,a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{backgrounds}

\begin{document}
    \begin{tikzpicture}[framed, background rectangle/.style={draw=black!15,fill=black!5,rounded corners=1ex}]
        \node (a) at (1,1) {a};
    \end{tikzpicture}
\end{document}

It produces:

Now I want to have an ellipse on the top darker line like:

My problem is that this ellipse should be predefined by some definition which gets at #1 some text to be drawn inside the ellipse. Finally I want to save the "style" (maybe its an option I couldn't get it by long research) as mybackground. So my aim is to write:

\documentclass[11pt,a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{backgrounds}

\begin{document}
    \begin{tikzpicture}[mybackground=#1]
        \node (a) at (1,1) {a};
    \end{tikzpicture}
\end{document}

To get the same result but now it should get #1 and write it in the ellipse.