Cook the pasta according to the instructions on the packet.
Meanwhile, heat a spoonful of oil in a frying pan and cook the garlic and chilli (if used) for a minute.
Rinse the spinach and add to the garlic. Squeeze some lemon juice over it, then turn the heat down and let the spinach steam for a couple of minutes. Stir in the crayfish.
Drain the pasta and stir in the spinach together with the lemon zest and mozzarella. Put the lid on, turn off the heat and let it steam for a couple of minutes to allow the cheese to melt.
Season with salt & pepper just before serving.