Heat a large frying pan over a medium-high heat and add the spinach with a tiny splash of water. Cook for 3-4 minutes or until completely wilted, then run under cold water. Once cold, squeeze out all the water. Chop finely and set aside.
In a saucepan, heat the milk and the stock together. In another saucepan, melt the butter over a medium heat, add the flour and cook for 2-3 minutes. When the mixture starts to turn brown, begin adding the milk and stock very slowly stirring constantly so it stays smooth. Then slowly add the cheese to the mixture, followed by the spinach - stir to combine. Season with salt, pepper and nutmeg to taste.