I see three ways to go about doing this: Go for something natural, but imperfect, like the Valles Marineris on Mars; cheat by using a small moon not in hydrostatic equilibrium, specifically a contact binary; or go for an artificial surface feature.
Valles Marineris is probably the most visually impressive rift valley in the solar system, being 4,000 km (2,500 mi) long, 200 km (120 mi) wide and up to 7 km (23,000 ft) deep and free of obscuring water or clouds it looks quite impressive. Rift valleys and structures suspected to be rift valleys have been found all over the solar system. Earth, Mars, Venus, Charon, Titania, Ariel, Oberon, Miranda, Tethys, and Luna have such structures.
While the martian Valles Marineris might be the best known and best looking example,
Ithaca Chasma on Thetys might be closest to what you want, as it is up to 100 km wide, 3 to 5 km deep and 2,000 km long and runs approximately three-quarters of the way around Tethys circumference.
I would also not disregard Eaths mid-ocean-ridges as they too are quite long.
Contact Binary Asteroid-Moon
The concept of hydrostatic equilibrium has also become important in determining whether an astronomical object is a planet, dwarf planet, or small Solar System body. According to the definition of planet adopted by the International Astronomical Union in 2006, one defining characteristic of planets and dwarf planets is that they are objects that have sufficient gravity to overcome their rigidity and assume hydrostatic equilibrium. Such a body will normally have the differentiated interior and geology of a world (a planemo), though near-hydrostatic bodies such as the proto-planet 4 Vesta may also be differentiated. Sometimes the equilibrium shape is an oblate spheroid, as is the case with Earth. However, in the cases of moons in synchronous orbit, nearly unidirectional tidal forces create a scalene ellipsoid. Also, the dwarf planet Haumea is scalene due to its rapid rotation. It had been thought that icy objects with a diameter larger than roughly 400 km are usually in hydrostatic equilibrium, whereas those smaller than that are not. Icy objects need less mass for hydrostatic equilibrium than rocky objects. The smallest object that appears to have an equilibrium shape is the icy moon Mimas at 397 km, whereas the largest object known to have a non-equilibrium shape is the rocky asteroid Vesta at 525 km (573 × 557 × 446 km). However, Mimas is not actually in hydrostatic equilibrium for its current rotation. The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet Ceres, at 945 km, whereas the largest body known to not be in hydrostatic equilibrium is the icy moon Iapetus, at 1,470 km. Because the terrestrial planets and dwarf planets (and likewise the larger satellites, like the Moon and Io) have irregular surfaces, this definition has some flexibility, but a specific means of quantifying an object's shape by this standard has not yet been announced. Local irregularities may be consistent with global equilibrium. For example, the massive base of the tallest mountain on Earth, Mauna Kea, has deformed and depressed the level of the surrounding crust, so that the overall distribution of mass approaches equilibrium. The amount of leeway afforded the definition could affect the classification of the asteroid Vesta, which may have solidified while in hydrostatic equilibrium but was subsequently significantly deformed by large impacts (now 572.6 × 557.2 × 446.4 km). - From Wikipedia on hydrostatic equilibrium
Essentially an object must have a certain mass so gravity can force it into a spherical form. Below this ill-defined limit odd geometries are possible. If you look at the pictures of 2014 MU69 (Ultima Thule), 67P/Churyumov–Gerasimenko or 25143 Itokawa, for example, you'll see examples of contact binaries. Two asteroids which meat each other and came so close that they touched. I imagine the upper plausible size limit for this would be to take 2 metallic asteroid like 16 Psyche with a diameter of roughly 200 km and slap them together. This would be a sizable moon which could be stable over geological timescales. The caveat would be that the gash won't be that much of an obstacle, one could probably jump across it with some effort. Why the gash is equatorial and not polar as it would be in the beginning could bee explained by later impacts.
One can get quite creative with reasons why a civilization would create a huge equatorial gash on a moon. Reasons could include
Because they can It is an art project or a demonstration of power.
Paraterraforming project The gash was created by excavation or detonations and then domed over to crate a moon-spanning garden-paradise.
Traces of a military operation Relativistic kill missiles, antimatter, gamma-ray lasers, star-powered particle beams, pick what you like to create a gash in a planet.
Mining It was a strip-mine. Down I the gash you'll find the remnants of a circumplanetary mass driver. This train to space used to export gigatons of minerals daily to support the economy of a K2 civilization. Maybe they were building a Dyson Swarm or some other megastructure which required such a strip-mine. One would expect to find many mineshafts branching out fro the central canyon, which was build to conveniently access the deeper mine-shafts.