How to Make It
Put the radishes in a heatproof bowl. In a small saucepan, combine the vinegar, sugar, salt and 1/4 cup of water and simmer until the sugar and salt are dissolved. Pour the warm brine over the radishes, weight them down with a plate and let stand for 10 minutes. Drain the radishes, reserving 2 tablespoons of the brine. Refrigerate the radishes until chilled.
In a small bowl, whisk the reserved radish brine with the mayonnaise, buttermilk, pepper and celery salt.
Roll the beef, cilantro and radishes in the lettuce leaves and transfer to a platter. Divide the buttermilk dunk between 6 small bowls and let guests dip the rolls into the mixture.