Pre-heat the oven to 200°C/400°F/gas mark 6. Put the onions, garlic, peppers, courgettes aubergines and passata into a large saucepan. Bring to a boil then cover and simmer for 10 minutes. Add the basil, season with salt and pepper and stir.
To make the cheese sauce
Put the butter, flour, mustard, milk and stock together in a pan. Heat gently and stir continuously until the sauce thickens. Don't worry about a few lumps. Remove from the heat and add about half the cheese and stir in until it melts.
Assembling the Lasagne
Spread a thin layer of the vegetable sauce on the bottom of the dish. Cover with a layer of uncooked lasagna sheets. Pour over a layer of cheese sauce and repeat this pattern until it is all used up. Sprinkle the remaining cheese over the top. If you are using a large dish, you may need more cheese.
Bake in the oven for 30 - 35 minutes.. Serve the lasagne with a green salad.
You will need a square or rectangular oven proof dish. I use a lasagne dish that is 26cm x 18cm (internal measurement) as I can fit 3 lasagna sheets across without having to break them. This is big enough for 4 people. You can of course use dishes that are other sizes. Try not to have lasagne sheets overlapping as they won't cook as well. If you need to break them, score them with a sharp knife first. Don't worry if it looks a bit messy, once the sauce is poured over them, they won't be seen.
You can make the basic sauce very quickly in just 10 minutes by simmering all the vegetables together in passata. If you have the time however, I would recommend cooking the sauce for longer in the oven at 180C/350F/gas mark 4 for 45 minutes to one hour. This really helps develop the flavours.
I use a basic white cheese sauce for this lasagne recipe. Add all the ingredients to a pan at the same time and keep stirring. Press out any lumps on the side of the pan with the back of your spoon. I always use a mature cheddar cheese in a white sauce as you need less of it to get that cheese flavour.
I like to use a mature cheddar cheese on the top as I think the flavour goes well. You can use other grated cheese such as mozarella which has a milder flavour but is more gooey when melted.
Ideally, defrost your lasagne before reheating. You can defrost it in the microwave on the defrost setting for about 5 minutes. Once defrosted, cover it with foil to stop the top from burning or browning further. Place in an ovenproof dish and reheat in the oven at 190°C / 375°F/ gas mark 5 for 20 - 30 minutes. The lasagne should be piping hot before serving. To check this, place a fork in the middle of the lasagne for a few seconds. If the fork is hot, the lasagne is also hot in the middle.
You can assemble this Mediterranean Vegetable Lasagne ahead of time and keep it in the fridge for 24 hours. Once cooked, it will keep in the fridge for 3 - 4 days. It can also be stored in the freezer for 2 - 3 months.