Heat the butter in a pan. Add the flour and cook, stirring with a whisk; the flour must not take on any colour. Remove the pan from the heat. Pour in all of the milk and cream at once, bring to the boil while stirring, then reduce the heat and simmer for approx. 10 mins., stirring occasionally until the sauce is thick and creamy.
Add the cheese to the béchamel sauce, melt, season and leave to cool.
Roll out the pasta dough, quarter lengthwise. Place 12 portions of the filling on the bottom half of each strip of pasta at equal intervals. Brush the upper halves of the pasta strips with a little egg, fold over the filling, press the edges down firmly, pressing out any trapped air. Separate the ravioli with a pastry wheel. Cook the ravioli in batches in boiling salted water for approx. 3 mins., drain, and keep warm.
Dry roast the walnuts in an uncoated frying pan until golden brown. Heat the butter in a pan until it foams and smells slightly nutty. Add the sage leaves, fry until crispy. Top the ravioli with the walnuts and sage butter. Sprinkle the reserved cheese over the top.