Drain the pasta, reserving a cup of water. Return the pasta and beans to the pan, and add the pesto, crème fraîche, prawns, lemon zest and juice, and pine nuts. Season well. If the sauce is a little thick, add a splash of the pasta water. When you’re ready to serve, scatter over the rocket. Eat hot, or leave to go cold, put in the fridge and eat as a pasta salad.