Place the peppers, onions, mushrooms and courgette into a large baking tray, coating with oil and seasoning. Roast for 25 minutes.
Mix the passata in a bowl with 2/3 of pesto.
Make the white sauce by melting the butter in a saucepan with flour, and cooking for 2 minutes. Gradually add and boil while stirring continuously. Lower heat and allow sauce to thicken before removing from flame.
Take a large oven dish and layer with the lasagna sheets. Add a layer of vegetables and pesto mixture. Add another layer of lasagna and then white sauce with cooked spinach placed on top.
Repeat the same process for multiple layers and then add remaining pesto, topped with cheese, and Parmigiano Reggiano.