The outburst of AT 2023txn, started in 2022 and still unfolding through multiple maxima and deep minima which bring the system well below quiescence brightness, has been monitored with BVRIgri photometry, low and high resolution spectroscopy, and Swift XRT+UVOT observations. AT 2023txn turns out to be a previously unknown symbiotic star, harboring an M6III red giant of great photometric stability during quiescence. A large and cold accretion disk around the white dwarf companion provided about 3/4 of the system brightness at blue wavelengths during quiescence, but was unable to power a significant emission-line spectrum as implied by the observations from the IPHAS riHalpha survey and Gaia DR3 BP-RP spectrum. The outburst begun toward the end of 2021, passed through a first unnoticed maximum in March 2022, and was eventually announced as transient Gaia23cse about 19 months later in September 2023 when the object was peaking at a second and brighter maximum. Close to that epoch we measured 1500 Lsun as the luminosity radiated by the outbursting component over the 2000-9000{AA} interval, with an upper limit of 1L_{sun}_ to the X-ray luminosity over the 0.3-10keV range. The emission-line spectrum has been characterized by persistent low ionization conditions, with FeII and Balmer series being the dominating species and varying their integrated flux in phase with the brightness evolution during the outburst. Three deep minima separated by about 866 days brought the system 1.5mag below the g-band brightness in quiescence, but they have no counterpart prior to the start of the outburst season. Interpreting them as eclipses of the hot component by the red giant encounters severe difficulties, and alternative explanations are considered in the framework of a massive accretion disk similar to T CrB which transfers mass to the WD only episodically via a collapse affecting the inner radii.