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Heat oven to 220C/200C fan/gas 7. Put the aubergine, courgette, pepper and onion in a large, shallow roasting tin. Scatter with the chilli flakes and season with pepper and a pinch of salt. Pour the oil over, toss together to coat (you can use your hands for this), then spread the vegetables out in a single layer. Roast for 15 mins, then give them a stir and spread them out again. Roast for another 10-15 mins until softened and turning golden.
While the vegetables are roasting, make the tomato sauce. Tip the tomatoes into a medium pan, then pour 100ml water into the empty can, swirl it round and pour into the saucepan. Add the carrot and garlic, and simmer gently for 20-25 mins, partly covered with the lid and stirring occasionally, until the carrot is just tender. Stir in the tomato purée and the lentils, season with pepper and simmer for 2 mins more. Remove from the heat, stir in the basil and set aside.
Tip the spinach into a large heatproof bowl, pour over boiling water, press the leaves into the water and leave for 30 secs. Drain in a large colander and rinse under cold water briefly to cool it down quickly. Squeeze the spinach to remove the excess water, then roughly chop. Beat the egg in a bowl and stir in the ricotta, oregano, nutmeg and some pepper.
Lay the lasagne sheets, spread well out, in a large shallow roasting tin and pour boiling water over to cover. Move them around with a wooden spoon to make sure they aren’t sticking, then leave for 5 mins. (‘No pre-cook’ lasagne sheets benefit from a brief soaking, which improves the texture and makes them more pliable for layering in the dish.) Drain well and immediately separate them out to prevent them sticking.
When the vegetables have finished roasting, lower the heat to 200C/180C fan/ gas 6. Spread a very thin layer of the tomato sauce (about 2 big spoonfuls) over the bottom of an ovenproof dish, about 25 x 18 x 7cm deep and 2 litres capacity. Lay 3 lasagne sheets across the width of the dish and spread over half of the remaining sauce and half of the roasted vegetables. Cover with 3 more lasagne sheets, then spread the spinach over the top and season with pepper. Put the ricotta mixture on top in spoonfuls and spread out over the spinach. Cover with the last 3 lasagne sheets. Spread over the rest of the tomato sauce and then the remaining roasted vegetables. Scatter over the mozzarella and Parmesan to cover the vegetables and sauce as much as possible, then top with the cherry tomatoes. Season with pepper and lay a piece of foil loosely over the top. Can be made up to a day ahead and chilled – allow an extra few mins when baking the next day. Or freeze for up to 1 month and thaw overnight in the fridge.
Bake for 35 mins, then remove the foil and bake for a further 5-10 mins until everything is bubbling around the edges. Scatter with oregano and basil leaves, and serve with a green salad.