Cut the liver into narrow strips. Clean the mushrooms and cut into small pieces. Finely dice shallot.
Sear the liver strips in 2 tablespoons of oil for 3-4 minutes and remove. Add the remaining oil, mushrooms, shallots and sage to the pan, turn over and fry for 5-7 minutes over high heat. Deglaze with the stock and cream and reduce until slightly creamy. Season with salt, pepper and lemon juice. Add the liver and briefly heat it up.
Cook the spaghetti in salted boiling water according to the package instructions, drain, drain and serve with the liver ragout.