Stew leeks in oil with a little salt until done. Stir in a generous amount of crème fraîche to make it creamy.
Fry some finely chopped shallots in oil in a frying pan. Add butter. Once the butter has stopped foaming fry the sliced mushrooms for a very short time. Sprinkle with salt and pepper.
Serve the leeks and the mushrooms over spaghetti.
With grated or slivered parmesan.