Heat the butter in a saucepan, then add the onion and cook for 5 mins until softened. Stir in the flour and cook for 2 mins, then slowly start to whisk in the milk. When it has all been incorporated, gently cook for 5 mins until the sauce has thickened.
Meanwhile, place the spinach in a large colander. Pour over a kettle full of boiling water until the leaves have wilted (you may have to do this twice). Place the spinach in a clean dishcloth, squeeze out any excess liquid, then roughly chop. Stir into the sauce with the cream, gently heat, then finely grate over some nutmeg and season well.