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Heat 25g butter in a saucepan, then add 1 finely chopped small onion and cook for 5 mins until softened.
Stir in 2 tbsp plain flour and cook for 2 mins, then slowly start to whisk in 200ml full-fat milk. When it has all been incorporated, gently cook for 5 mins until the sauce has thickened.
Meanwhile, place two 200g bags 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 100ml single cream, gently heat, then finely grate over some fresh nutmeg and season well.