Cook the pasta shapes for 8–10 minutes or according to packet instructions until just tender. Drain and set aside.
Meanwhile, place the spinach in a pan of boiling water and cook, stirring continuously, over a moderate heat for 2 minutes until wilted. Remove from the heat and drain well, then return to the pan and toss with the nutmeg. Set aside.
Melt the butter in a medium, heavy-based saucepan. Remove from the heat, add the flour and stir to form a thick paste. Return to the heat and cook gently for a few seconds, stirring continuously. Remove from the heat and gradually add the milk, stirring well after each addition. Return to the heat and bring to the boil, stirring continuously until the sauce has boiled and thickened.
Remove from the heat and add the spinach and cheese. Stir, then transfer to a food processor and whiz until smooth. Return to the pan and add the pasta. Stir well to coat, then divide between 4 warmed serving bowls.