Cook the pasta in a large pan of boiling water until al dente. Drain.
Meanwhile, slice the onion, leeks and fennel. Add to a large saute pan with the butter and cook for 10 minutes until softened. Halve the mushrooms and add these – cook for a further 5 minutes.
Stir in the cream and 200ml water. Add the oregano, bring to a simmer then season with salt and pepper.
Mix the sauce with the pasta and leave to sit for 2 minutes. Enjoy!