Fill a large bowl with water, then cut sweet potatoes into fries, peeling them first if desired. While cutting, quickly add fries to water bowl to prevent browning. Place sweet potato fries, garlic cloves, dried sage, and lemon zest in fermentation jar.
Dissolve kosher salt in water to make a brine. Pour brine over potatoes, add a fermentation weight, and make sure the fries are completely submerged. Cover with an air lock lid or a loosely fitted lid.
Place in a dark spot in a cupboard or pantry. Ferment for about 3 days, depending on room temperature.
Drain fries, discarding garlic, sage, and lemon zest, then pat fries thoroughly dry before cooking.