If you use frozen spinach, defrost it in the microwave for a few minutes at 300 Watt or take it out for a few hours to defrost in the open air.
Boil the pasta water. Once the water boils, cook the spaghetti as indicated on the package minus 1 minute.
In the meantime, slice the mushrooms.
Heat 2 to 3 spoonfuls of olive oil in a frying pan. Add the mushrooms. Cook for about 1-2 minutes, then add the spinach leaves.
Add the chopped garlic clove. Then, after about 3 minutes, add the crème fraiche. Season with salt and pepper.
Shortly before the spaghetti finishes cooking, collect 10 cl of the pasta water. Add it to the spinach and mushroom sauce.