Melt the butter in a saucepan. Add flour and stir to mix the butter and the flour. Add the milk little by little by stirring with a whisk until the flour starts to thicken. Add White pepper and Castello® extra creamy brie. Mix everything together with a ladle until you have a thick cheese sauce.
Cook the pasta al dente and drain the water. Cut the champignons in slices and fry them golden in a little butter. Mix champignons together with the cheese sauce and basil. Pour over the tagliatelle. Sprinkle a bit of fresh spring onion and serve.