Place the spinach in a large colander in a clean sink.
Carefully pour boiling water over the spinach. Protecting your hands with oven mitts, shake the hot colander and leave the spinach to drain.
Heat the butter in a large saucepan over medium heat.
Sauté the onion until it is just beginning to soften.
Sprinkle the buttery onion with the flour and stir through.
Cook for a minute or two, stirring constantly.
Gradually whisk in the milk, a bit at a time, stirring constantly. Allow the sauce to thicken a little between additions.
Place the colander containing the spinach on a tea towel nearby.
Pick up the spinach in clean hands, a bit at a time, squeezing out any remaining water before stirring it into the cream sauce.
When all the spinach has been added, gradually stir in the cream until a smooth, thick consistency is reached. You many not need it all, or you may need a little more. You want the spinach to be in a nice creamy sauce, but you don't want it to run all over the plate.
Add salt, pepper and nutmeg to taste.
Transfer to a serving dish and serve immediately.