Preheat your oven to 180 °C/356° F, Fan. This initial step is crucial as it ensures your lasagna cooks evenly from the start. A hot oven is essential for achieving that golden, bubbly top we all crave.
In a warm pan over medium heat, add the finely chopped onion and spinach. Sauté them together until the liquid has evaporated, leaving you with flavorful vegetables. You'll know it's ready when the spinach is wilted and the onion is translucent.
In a separate saucepan set over medium heat, melt the butter. Once it’s completely melted, add the flour. Stir with a hand whisk, cooking until you see a light golden color. This roux is the foundation for your creamy sauce.
Gradually add the milk to the roux in batches, stirring constantly with a whisk to avoid lumps. Keep stirring until the mixture thickens and turns creamy, which usually takes around 5 to 7 minutes. You want a velvety consistency that coats the back of a spoon.
Now, stir in the sautéed spinach, pesto, Parmesan, salt, and pepper. Mix everything until you have a uniform filling that smells incredible! Set this mixture aside as it will be layered into the lasagna.
Take a baking pan (25×35 cm/10×14 inches works perfectly) and spread a thin layer of your cheese cream on the bottom. This prevents the first layer of lasagna sheets from sticking.
Layer three lasagna sheets over the sauce, then add another layer of the creamy filling. Continue layering like this until you run out of lasagna sheets and filling, finishing with a layer of cream on top. Make sure the top layer is well coated for that delicious crust!
Sprinkle extra Parmesan on top to enhance the flavor and color. Now, it’s time to bake! Place the pan in the preheated oven and bake for 35 to 40 minutes. You’ll know it’s done when the top is golden and bubbly.
Once baked, allow the lasagna to cool for a few minutes before cutting into it. This resting time allows everything to set so you can serve it up without it falling apart!