- In a bowl, whisk together the lemon juice, mustard and egg yolk. While whisking constantly, add the oil in a thin stream. Season with salt and pepper. Set aside.
- Scoop out the seeds from the cantaloupe, slice it thinly and remove the rind. Lay overlapping melon slices on four plates.
- Brush the melon slices with the dressing. Garnish with prosciutto rosettes and watercress and season with a few grindings of pepper.