  We report the discovery of SRGE J075818-612027, a deep stream-eclipsing
magnetic cataclysmic variable found serendipitously in SRG/eROSITA CalPV
observations of the open cluster NGC~2516 as an unrelated X-ray source. An
X-ray timing and spectral analysis of the eROSITA data is presented and
supplemented by an analysis of TESS photometry and SALT spectroscopy. X-ray
photometry reveals two pronounced dips repeating with a period of $106.144(1)$
min. The 14-month TESS data reveal the same unique period. A low-resolution
identification spectrum obtained with SALT displays hydrogen Balmer emission
lines on a fairly blue continuum. The spectrum and the stability of the
photometric signal led to the classification of the new object as a polar-type
cataclysmic variable. In this picture, the dips in the X-ray light curve are
explained by absorption in the intervening accretion stream and by a
self-eclipse of the main accretion region. The object displays large magnitude
differences on long (months) timescales both at optical and X-ray wavelengths,
being interpreted as high and low states and thus supporting the identification
as a polar. The bright phase X-ray spectrum can be reflected with single
temperature thermal emission with 9.7 keV and bolometric X-ray luminosity
$L_{\rm X} \simeq 8\times 10^{32}$erg s$^{-1}$ at a distance of about 2.7 kpc.
It lacks the pronounced soft X-ray emission component prominently found in
ROSAT-discovered polars.
