To make the pangritata, lightly toast the sourdough. Peel the garlic, then blitz in a food processor with the toasted bread, herbs and walnuts until fine. Preheat a non-stick frying pan over a medium heat, add a drizzle of olive oil, then add the pangritata and fry for around 5 minutes, or until golden and crisp.
Meanwhile, bring a large pan of salted water to the boil, then reduce the heat to low and place a large heatproof bowl on top. Add the mascarpone or crème fraîche to the bowl along with the leftover cheeses, crumbling or grating any harder varieties first, then stir until melted. Carefully remove the bowl from the heat and set aside, then turn the heat up to high. Once the water is boiling, add the pasta and cook according to packet instructions. Drain, reserving a cupful of cooking water, then add to the bowl of melted cheese and toss to coat, adding a splash of the reserved cooking water to loosen, if needed.