Melt the butter in a heavy-based saucepan, add the onion and sauté until soft and translucent (about 4 minutes). Add the cream and bring to the boil. Reduce the heat to low and simmer until the cream is reduced by half. Add the spinach and blend roughly with a stick blender, thinning the mixture with milk until the desired consistency is achieved (mine is the consistency of thick custard). Season to taste with nutmeg, flaky salt and freshly ground white pepper. Serve in a bowl.