Cook the pasta following pack instructions. Meanwhile, toast the walnuts in a dry pan for a few mins. Add half the walnuts to the small bowl of a food processor or a hand chopper, along with the red peppers, Parmesan, garlic, basil, oil and some seasoning. Whizz to a paste, adding a splash of water from the pasta if it is a little dry.
Drain the pasta, reserving a cup of the cooking water. Return the pasta to the pan and set over a low heat. Add the pesto, mascarpone and 3-4 tbsp of the reserved pasta water, then stir until the mascarpone has melted, adding a splash more pasta water if the sauce needs thinning. To serve, crush the remaining walnuts in your hand and scatter over the pasta with a few more basil leaves and some extra Parmesan.