Mix the mayonnaise, honey and mustard until smooth. Spread the slices of bread with the mayonnaise mixture.
Lay 3 slices of bread, mayonnaise-side up, and put some slices of roast beef on each.
Roughly chop the watercress and scatter over the beef, then season with salt and pepper.
Put the tops on the sandwiches and cut into triangles, fingers or squares, as you like.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.