Put the pasta on to boil in well-salted water. Cook until very al dente.

While the pasta is cooking, make your velouté. Sauté the chopped celery in the butter, in a large saucepan, until it begins to soften. Add the flour and continue to sauté for a minute or two. Then add the hot broth, stirring vigorously with a whisk. As the broth comes to a simmer, it should thicken into a creamy sauce. Let the sauce simmer for 2-3 minutes, then remove it from the heat. Whisk in the sour cream until the mixture is perfectly smooth and homogenous. Taste and adjust for seasoning.

Also while the pasta is cooking, sauté the mushrooms in the olive oil. Add a pinch of salt along the way; this will help the mushrooms give off their liquid. Once the liquid has evaporated and the mushrooms begin to brown, they should be done.

Once the pasta is cooked, drain it well and pour it into a large mixing bowl. Add the velouté, cooked turkey and the sautéed mushrooms, and mix everything together well.

Pour the mixture into a well-buttered baking dish. Top with grated Parmesan cheese, some breadcrumbs if you like and dot bits of butter here and there.

Bake in a moderate (180C/350F) oven for about 45 minutes, or until the dish is bubbling and the top is nice and brown. Most (but not quite all) of the sauce should have evaporated. Let the dish rest for 15-20 minutes before serving.