Put the radishes, vinegar, sugar and a good pinch of salt in a bowl and leave to pickle for 30 mins, or up to 24 hrs.
When you’re ready to serve, toast the bagels while you mix together the crème fraîche and mustard. To serve, spread the bagels with the mustardy crème fraîche, top each half with a ruffled slice of beef, a few pickled radishes and a few watercress sprigs. Finish with a grind of black pepper.