  Observations of synchrotron emission at low radio frequencies reveal a
labyrinth of polarised Galactic structures. However, the explanation for the
wealth of structures remains uncertain due to the complex interactions between
the interstellar medium and the magnetic field. A multi-tracer approach to the
analysis of large sky areas is needed. This paper aims to use polarimetric
images from the LOFAR Two metre Sky Survey (LoTSS) to produce the biggest
mosaic of polarised emission in the northern sky at low radio frequencies (150
MHz) to date. The large area this mosaic covers allows for detailed
morphological and statistical studies of polarised structures in the
high-latitude outer Galaxy, including the well-known Loop III region. We
produced a 3100 square degree Faraday tomographic cube using a rotation measure
synthesis tool. We calculated the statistical moments of Faraday spectra and
compared them with data sets at higher frequencies (1.4 GHz) and with a map of
a rotation measure derived from extragalactic sources. The mosaic is dominated
by polarised emission connected to Loop III. Additionally, the mosaic reveals
an abundance of other morphological structures, mainly {narrow and extended}
depolarisation canals, which are found to be ubiquitous. We find a correlation
between the map of an extragalactic rotation measure and the LoTSS first
Faraday moment image. The ratio of the two deviates from a simple model of a
Burn slab (Burn 1966) along the line of sight, which highlights the high level
of complexity in the magnetoionic medium that can be studied at these
frequencies.
