Cook the spaghetti in salted water until al dente. Set aside a cupful of the cooking water.
Clean and wash the mushrooms, then slice them. Set aside.
Finely chop the onion. Set aside.
Wash the parsley and chop it finely. Set aside.
Heat a tablespoon of olive oil in a high-sided frying pan, add the onion and fry until translucent. Add the sliced mushrooms and fry.
Season with salt and pepper. Add the parsley.
When the liquid has completely evaporated, sprinkle in a tablespoon of oat flour. Stir and fry lightly, then pour in half a cup of cooking water. Stir until thickened. Add the cream. Stir again. Add the rest of the cooking liquid. Adjust the consistency to your liking.
Add salt and pepper if necessary.
Stir in the pasta and it's ready to serve.