Scrub and peel the radishes. Grate on the coarse side of a grater; you should have 4 cups. Toss with the coarse salt and place, loosely covered, in the refrigerator for 4 hours. Remove, place in a strainer, and rinse under very cold running water. Pick up handfuls of the radish and squeeze out as much water as possible. At that point you should have about 2 cups of compacted radish. Peel and grate the onion and toss with the radish, along with the chicken fat and pepper.

Pack into a clean jar, cover tightly, and place in the refrigerator for at least 4 days before eating. The relish will last for weeks, so double or triple this amount if you like the results.