Preheat your oven to 200ºC.
Heat 20cl of cream with the stock cube in a saucepan.
Once the stock cube is dissolved, add the frozen spinach and the remaining 20cl of cream. Cover and leave to cook gently for 6 to 7 minutes, mixing regularly. If ever the spinach mixture gets too thick add a little bit of milk.
In a lasagna dish (20 x 25 cm), pour a little bit of the spinach mixture first so as the first layer of lasagna does not stick.
Build the lasagna going: lasagna sheets, spinach mixture, grated cheese, lasagna sheets... finishing with a little bit of the spinach mixture (to ensure your last layer of lasagna sheet is moist) and grated cheese.
Put in the oven for 30 minutes.