Preheat the oven to 180°C (356°F) fan.
Heat a large frying pan over medium heat and add a splash of olive oil.
Add the finely chopped onion and the coarsely chopped spinach and sauté, stirring occasionally, until the liquids have evaporated; remove from heat and set aside.
In a saucepan over medium heat melt the butter, then add the flour and whisk continuously for 1–2 minutes to cook the flour.
Add the milk in batches while whisking constantly to prevent lumps, and cook until the béchamel thickens to a creamy consistency.
Stir the sautéed spinach and onion into the béchamel, then add the basil pesto and 100 g grated Parmesan; season with salt and pepper and mix until uniform.
Spoon a little of the cheese cream into the bottom of a 25×35 cm (10×14 in) baking pan.
Place 3 lasagna sheets over the cream, spread more cream on top, and continue layering sheets and cream until the ingredients are used up.
Sprinkle extra Parmesan over the top and bake for 35–40 minutes, until golden and bubbling.
Allow the lasagna to cool slightly, then cut into portions and serve.