Cook the pasta according to packet instructions.
Meanwhile, heat a little oil in a frying pan and cook the garlic for a 30 seconds over low heat, then add the anchovies (and chilli flakes if used). Cook for another minute or so until you get a paste.
Rinse the spinach and add to the pan with the lemon juice and a spoonful of the pasta’s cooking water. Season with salt and pepper and mix well. Put a lid on and let the spinach steam over low heat.
Take off the lid after 3 minutes and stir until the spinach has completely wilted. Keep it warm under the lid until the pasta is ready.
Drain the pasta and mix with spinach sauce.
Add some grated cheese, if preferred, and serve.