Heat oven to 200C/180C fan/gas 6. Put the pine nuts in a dry frying pan, gently toast for 2-3 mins, then tip onto a plate. In the same pan, add the pancetta or bacon and cook for a few mins until crisp.
Bring a large pan of salted water to the boil and cook the pasta following pack instructions until al dente. Drain, reserving a ladleful of the pasta cooking water for later.
Meanwhile, put the double cream, crème fraîche and half the cheese in a saucepan, stir together and gently warm through until the cheese has melted. Take off the heat, stir in the mustard and some seasoning to taste.
Tip the drained pasta into a large baking dish and stir in the cheese sauce, adding some of the pasta cooking water if it looks a little dry. Sprinkle the toasted pine nuts and cooked pancetta over the top, then scatter over the remaining cheese. Bake for 20 mins until the top is golden and bubbling, then serve with a salad, if you like.