Pre-heat oven to 200°C/Fan 180°C/Gas Mark 6. Place the peppers, onions, mushrooms and courgette into a large baking tray, and coat in olive oil and season. Roast the vegetables for 25 minutes.
In a bowl mix the passata with 2/3 of the pesto.
Prepare the béchamel sauce by melting the butter in a saucepan, add the plain flour and gently whisk and cook for 2 minutes. Gradually add the milk and then bring to the boil, stirring continuously. Reduce the heat, and let the sauce thicken, and then remove from the heat.
Lightly oil an ovenproof dish and place a layer of lasagne sheets, spoon over half the vegetables then top with a third of the passata and pesto mixture. Add a further layer of lasagne sheets and top with half the béchamel, then scatter the cooked spinach on top. Top again with lasagne sheets and then the remaining vegetables and another third of the passata. Finish with the remaining lasagne sheets, béchamel, and passata. Drizzle the remaining pesto and top with slices of mozzarella, then scatter the Parmigiano Reggiano over the top. Bake in the oven at 200°C/fan 180°C/Gas Mark 6 for 20 minutes.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.