Heat the oil in a large pan over a medium heat and fry the onion for 5 mins, until soft. Add the courgettes, aubergines and peppers. Season, then fry for 10 mins, stirring often, until lightly golden and softened. Add the garlic and fry for 1-2 mins, until fragrant. Stir in the herbs, tomatoes and stock and simmer, uncovered, for 20 mins, until lightly thickened.

Meanwhile, make the bechamel. Melt the butter in a medium pan, then stir in the flour and cook for a minute over a low heat. Add the milk a splash at a time, whisking constantly, until you have a thick, smooth white sauce. Season.

Heat the oven to 200C/fan 180C/gas 6. In a large baking dish, layer up a spoon of the ratatouille, a spoon of bechamel (keep a third for the top), and a layer of lasagne sheets. Repeat the layers until the ratatouille is used up. Spoon over the reserved bechamel, then top with the mozzarella and parmesan. Bake for 40-50 mins until golden and bubbling, and a cutlery knife slides easily into the middle. Leave to stand for 10 mins, then serve with salad and garlic bread.