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Heat the oil in a medium saucepan over a medium heat and fry the onion and mushrooms for 10 mins, or until softened and browned slightly. Meanwhile, cook the pasta following pack instructions.
Add the garlic to the pan with the mushrooms and cook for 2 mins more. Tip in the crème fraîche and parmesan, stir to combine, then add the baby spinach. Set aside.
Remove the pasta from the heat and drain, reserving 25ml of the water. Toss the pasta in the creamy mushroom sauce, put back on the heat and cook over a low heat until the spinach wilts, about 5 mins. Pour in enough of the reserved water to loosen slightly. Season with black pepper and finish with a sprinkle of chilli flakes, if you like.