Heat the oil in a pan and gently fry the spring onions and garlic for a few minutes, or until pale golden-brown. Add the mushrooms and fry over a high heat for 8-10 minutes, or until the water has evaporated. Tip the spinach into the pan, and stir regularly as it wilts. Continue cooking over a medium heat until most of the water has evaporated.
Sprinkle the flour over the mixture, stir it in well, then cook for a further minute. Remove the pan from the heat and stir in the crème fraîche. Return the pan to the heat, bring to the boil, and simmer for a few minutes, or until the sauce has thickened. Season to taste with salt and freshly ground pepper.
Meanwhile, cook the pasta in a large pan of salted boiling water, according to packet instructions. Once cooked, drain the pasta, and toss with the mushroom and spinach sauce. Serve immediately.