Trim the kohlrabi of their leaves and stems, then peel them. Slice them thinly into matchsticks about 2 inches long by 1/8 inch thick. Place them in a large mixing bowl.

Remove and discard the long stalks of the fennel bulbs, but reserve about 1/2 cup of loosely packed fronds. Chop the fronds finely, and then slice the fennel bulbs no thicker than 1/8 inch. Add the sliced bulbs and chopped fronds to the bowl with the kohlrabi.

Toss the apples and salt into the bowl and knead the ingredients with your hands until they release their juice. Layer the mixture into a quart-sized fermentation crock, 1 cup at a time. Pack it tightly into the crock so that any air escapes and the ingredients release more of their juice. Continue packing and layering until you’ve added the entire contents of the mixing bowl to the crock. Pack down the ingredients once more to ensure that they are completely submerged in the brine and that the brine rests below the lip of the crock by at least 1 inch.

Close the crock and ferment at room temperature for 10 to 14 days before tasting. If you prefer a stronger or sourer flavor, continue fermenting until done to your liking, testing every 3 to 5 days. Transfer to the refrigerator, root cellar, or other place of cold storage once the relish achieves the level of sourness you prefer and use within 6 months.