• Preheat the oven to 180 ° C/356° F, Fan.
• Warm a pan over medium heat, add the onion and spinach, and sauté until the liquids have evaporated and set aside.

For the cheese cream:

• Place a saucepan over medium heat and add the butter. Once melted, add the flour, stir with a hand whisk, and sauté until the flour is cooked.
• Add the milk in batches, stirring constantly with a whisk to avoid lumps and cook until the cream thickens.
• Add the spinach, pesto, Parmesan, salt and pepper, and stir until the mixture is uniform, then set aside.
• Spread a little of the cheese cream on the bottom of a 25x35 cm (10x14") baking pan, top with 3 lasagna sheets and top with cream. Layer like this until the lasagna and cream are used up.
• Sprinkle with Parmesan and bake for 35-40 minutes.
• Allow the lasagna to cool, cut and serve.