You can purchase shredded cabbage at the store, or shred it yourself using a mandoline or sharp knife. To do that, slice the cabbage in half (or buy 1/2 cabbage instead of a whole head), then quarter it. Run two of the quarters through a mandoline, using the guard, or thinly slice the quarters with your knife until you get ~6 cups.
Slice the radishes (long, French breakfast radishes are preferred, but you can use round ones too) into matchsticks. Add the cabbage and radishes to a large bowl.
In a small bowl, whisk together the Greek yogurt, apple cider vinegar, honey, garlic, and celery salt. Pour this over the cabbage and radishes and mix until they are coated.
Serve immediately as a side to barbecue or burgers, on sandwiches or tacos, or with grilled fish.