  Spatially resolved studies of high redshift galaxies, an essential insight
into galaxy formation processes, have been mostly limited to stacking or
unusually bright objects. We present here the study of a typical (L$^{*}$,
M$_\star$ = 6 $\times 10^9$ $M_\odot$) young lensed galaxy at $z=3.5$, observed
with MUSE, for which we obtain 2D resolved spatial information of Ly$\alpha$
and, for the first time, of CIII] emission. The exceptional signal-to-noise of
the data reveals UV emission and absorption lines rarely seen at these
redshifts, allowing us to derive important physical properties (T$_e\sim$15600
K, n$_e\sim$300 cm$^{-3}$, covering fraction f$_c\sim0.4$) using multiple
diagnostics. Inferred stellar and gas-phase metallicities point towards a low
metallicity object (Z$_{\mathrm{stellar}}$ = $\sim$ 0.07 Z$_\odot$ and
Z$_{\mathrm{ISM}}$ $<$ 0.16 Z$_\odot$). The Ly$\alpha$ emission extends over
$\sim$10 kpc across the galaxy and presents a very uniform spectral profile,
showing only a small velocity shift which is unrelated to the intrinsic
kinematics of the nebular emission. The Ly$\alpha$ extension is $\sim$4 times
larger than the continuum emission, and makes this object comparable to
low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies
and Ly$\alpha$ emitters usually studied at high redshift. We model the
Ly$\alpha$ line and surface brightness profile using a radiative transfer code
in an expanding gas shell, finding that this model provides a good description
of both observables.
