Line a sieve with cheesecloth. If the cheesecloth is loosely woven (most supermarket brands are), use a triple thickness. Pour the yogurt into the sieve, then gather the edges of the cheesecloth and tie into a bag. Hang the bag over the sink or over a bowl to catch the drippings. Let drain for 8 hours.

Turn the soft, creamy cheese out onto a platter. Season the surface with salt to taste, drizzle with olive oil, then sprinkle generously with za'atar. Surround the cheese with the olives and radishes. Serve with the warm pita bread wedges.