Thaw the spinach and squeeze it out well. Rub the lemon peel and squeeze the lemon. Heat the broth in a saucepan.
Sweat the onion cubes in half of the butter until translucent, add rice and sauté until the grains start to shine.
Gradually add the stock with a ladle, when the liquid is soaked up, add the next one at a time. Mix in the lemon juice and the zest. Cook the rice while stirring until it is soft (approx. 20 to 25 minutes), add more stock if necessary.
Finally add the prawns, spinach and remaining butter to the rice, remove from heat, stir in the parmesan, season with salt and pepper and serve.