To make the dressing, mash the anchovies, garlic and a little white pepper in a mortar with a pestle. Slowly pound in the olive oil, lemon juice and then, finally, the herbs until you have a thick green slush.
Check the seasoning. In a bowl, gently toss the radicchio with one-third of the dressing then arrange the leaves in the centre of a large platter or eight plates.
Place the beans, prawns and remaining dressing in the bowl and toss gently. Pile onto the platter or divide among the plates and sprinkle with sea salt and a grind of white pepper.