Slice the mushrooms and garlic. Add the oil and garlic to a large frying pan.
When the oil is hot and garlic is sizzling, add the mushrooms and stir for a minute. Add a couple of pinches of chilli flakes, if used.
Cover the pan with a lid, turn down the heat and cook for 5 minutes.
Add zest and juice of the lemon. Put the cover on again and cook until the mushrooms have softened and released water. This should take 10-15 minutes over low heat. Season with salt and pepper (and a pinch of ground nutmeg if liked). Turn up the heat and reduce the sauce by half, then add the spinach, cover, and turn off the heat.
Let it rest for 5 minutes until the spinach has wilted, then combine well.
Serve with some chopped parsley and some extra chilli flakes, if you like.