Cook the pasta in a large saucepan of salted boiling water according to the packet instructions until al dente.
Meanwhile, heat a small, dry frying pan over a medium heat, add the mushrooms and cook, stirring, for 5 minutes. Add the garlic and cook, stirring, for 1 minute. Add the cream, milk and nutmeg and bring just to boiling point.
Stir in 125 g (4 oz) of the cheese and all the basil, remove from the heat and stir until the cheese has melted. Season with salt and pepper.
Drain the pasta and tip into a large bowl. Add the sauce and stir well to combine.
Spoon into individual gratin dishes, top with the remaining cheese and bake in a preheated oven, 230°C (450°F), Gas Mark 8, for 10 minutes until golden brown. Serve immediately.