Boil the pasta according to pack instructions. In a saucepan, melt the butter, then gently fry the onion until golden. Add the garlic and sage, fry for a further 2 mins, then remove the pan from the heat. Drain the pasta and reserve some cooking water. Stir through the buttery onions, stilton and 2 tbsp cooking water, then sprinkle with the toasted walnuts to serve.