Soak the dried mushrooms in 250 ml boiling water for 30 minutes.
Heat the olive oil and butter in a pan and cook the leek for 5 minutes. Add the carrots, celery, thyme and parsley and cook for five minutes more, then add the beef and cook, stirring, until nicely browned.
Scatter over the flour, sea salt and pepper and stir for 1-2 minutes to cook the flour.
Add the mushrooms and their soaking water, discarding any sediment. Gradually stir in the hot stock or water then simmer, partly covered, for 45 minutes or longer.
Cook the spaghetti in plenty of boiling salted water until al dente. Drain well, then toss with the meat sauce and add the grated lemon zest. Serve with freshly grated parmesan.