\parbox in mathmode in Lyx

Question

I am trying to write multiline text inside the mathmode in Lyx like this:

I did that based on this answer. I have made it work in Lyx by defining a macro

so that the usage is

and the output is that of the first picture. But there are some problems:

1. As you can see I have to introduce blanc spaces "by hand" typing "\ ". I would like to be able to introduce spaces just hitting the bar (the same way as when inside a \text box).

2. There is no way to introduce mathematical symbols. For example if instead of "This is a nice function..." I want to write "f(x) is a nice function...".

3. I would like to have the possibility to introduce the characters áéíóú because I typically write in Spanish. This is also being impossible with my definition.

EDIT:

Answering to PhilipPirrip comment:

produces

Answer 1

I do not know LyX, but here are two possible LaTeX codes, based on empheq(which loads \=mathtools, which loads amsmath:

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{empheq}
\usepackage{bigstrut}

\begin{document}

        \[ f(x)\to\begin{dcases}\parbox{40mm}{$f(x)$ is a nice \emph{función}, so I want to add this multiline comment\bigstrut[b]}
            \end{dcases} \]%

        \begin{empheq}[left=f(x)\to\empheqlbrace]{equation*}
          \parbox{40mm}{$f(x)$ is a nice \emph{función}, so I want to add this multiline comment}
        \end{empheq}

\end{document}