Wash celery root then set on cutting board. Cut one side of the celery root off and place cut side down on cutting board. Continue to trim away the rough edges of celery root into an approximate cube shape. Rinse cutting board and celery root again to remove any dirt or outer fibers that may cling to either. Clean up trimmed celery root with a peeler or a small knife to remove any remaining rough edges. Using a mandoline, thinly slice celery root into 1/16-inch slices. Cut lemon in half and squeeze into a bowl of water. Place celery root slices in lemon water and set aside.